Nickelocene Molecules Research Articles

Single-Spin Sensing: A Molecule-on-Tip Approach.

Magnetometry plays a pivotal role in addressing the requirements of ultradense storage technology and overcoming challenges associated with downscaled spin qubits. A promising approach for atomic-scale single-spin sensing involves utilizing a magnetic molecule as a spin sensor, although such a realization is still in its early stages. To tackle this challenge and underscore the potential of this method, we combined a nickelocene molecule with scanning tunneling microscopy to perform versatile spin-sensitive imaging of magnetic surfaces. We investigated model Co islands on Cu(111) of different thicknesses having variable magnetic properties. Our method demonstrates robustness and reproducibility, providing atomic-scale sensitivity to spin polarization and magnetization orientation, owing to a direct exchange coupling between the nickelocene-terminated tip and the Co surfaces. We showcase the accessibility of magnetic exchange maps using this technique, revealing unique signatures in magnetic corrugation, which are well described by computed spin-density maps. These advancements significantly improve our capacity to probe and visualize magnetism at the atomic level.

Unveiling the Decisive Factor for the Sharp Transition in the Scanning Tunneling Spectroscopy of a Single Nickelocene Molecule.

Scanning tunneling microscopy (STM) has been utilized to realize the precise measurement and control of local spin states. Experiments have demonstrated that when a nickelocene (Nc) molecule is attached to the apex of an STM tip, the dI/dV spectra exhibit a sharp or a smooth transition when the tip is displaced toward the substrate. However, what leads to the two distinct types of transitions remains unclear, and more intriguingly, the physical origin of the abrupt change in the line shape of dI/dV spectra remains unclear. To clarify these intriguing issues, we perform first-principles-based simulations on the STM tip control process for the Cu tip/Nc/Cu(100) junction. In particular, we find that the suddenly enhanced hybridization between the d orbitals on the Ni ion and the metallic bands in the substrate leads to Kondo correlation overwhelming spin excitation, which is the main cause of the sharp transition in the dI/dV spectra observed experimentally.

Molecular Dynamics and Surface Interactions of Nickelocene Adsorbed on Silica: A Paramagnetic Solid-State NMR Study.

When grinding nickelocene with silica in the absence of a solvent at room temperature, it adsorbs on the surface within the pores. This has also been demonstrated visually by adsorbing green nickelocene in the pores of a large colorless silica gel specimen. While this dry adsorption and translational mobility of nickelocene within the pores is proven visually, the site-to-site mobility of the nickelocene molecules and their orientation toward the surface are not yet understood. In this contribution, mesoporous silica is used as the support material for a systematic solid-state NMR study of these issues. Paramagnetic 1H VT solid-state NMR and T1 relaxation times have been powerful tools for studying the dynamics of nickelocene on the silica surface. Herewith, the mobility of the surface-adsorbed nickelocene molecules in the pores could be quantified on the molecular scale. According to the obtained data, the nickelocene molecules move like a liquid on the surface. Isotropically moving molecules exchange places rapidly with surface-attached molecular states of nickelocene in a sample with submonolayer surface coverage. This finding is corroborated by a macroscopic visualization experiment. The states of the surface-attached horizontally oriented nickelocene molecules that are prevalent at temperatures below 200 K have been quantified. The temperature dependencies of the rate k in coordinates of ln(k) versus 1/T and ln(k/T) versus 1/T form ideal straight lines that allow the determination of the kinetic parameters Eact = 5.5 kcal/mol, A = 1.1 × 1010, ΔH‡ = 5.0 kcal/mol, and ΔS‡ = -15 eu. Investigating a sample with equal amounts of nickelocene and ferrocene in a submonolayer amount of 80% overall surface coverage shows that the different metallocenes mix on the molecular level on the silica surface.

Unravelling the robustness of magnetic anisotropy of a nickelocene molecule in different environments: a first-principles-based study.

Recent scanning tunneling spectroscopy with single metallocene molecule-functionalized tips have proved to be a powerful tool to probe and control individual spins and spin-spin exchange interactions due to the robustness of the magnetic properties of the metallocene molecule in different surroundings. However, accurate prediction of such robustness at a first-principles-based level by the conventional density functional theory (DFT) has remained challenging. In this paper, we have performed a detailed investigation of the evolution of electronic and magnetic properties of a nickelocene molecule (NiCp2) in different environments, i.e., free-standing, adsorbed on Cu(100) and as a functionalized tip apex. Using an embedding method, which combines DFT and the complete active space self-consistent field (CASSCF) method recently developed, we demonstrate that the nickelocene molecule almost preserves its spin and magnetic anisotropy upon adsorption on Cu(100), and also in the position of the tip apex. In particular, the cyclic π* orbital of the Cp rings could hybridize with the singly occupied dπ orbitals of the Ni center of the molecule, protecting these orbitals from external states. Hence the molecular spin maintains S = 1, the same as in the free-standing case, and its magnetic anisotropy is also robust with energies of 3.56, 3.34, and 3.51 meV in free-standing, adsorbed on Cu(100), and functionalized tip apex states, respectively, in good agreement with previous theoretical and experimental results. This work thus provides a first-principles-based understanding of the relevant experiments. Such agreement between theoretical simulations and experimental measurements highlights the potential usefulness of the method for investigating the local electronic and spin states of organometallic molecule-surface composite systems.

Nickelocene-Filled Purely Metallic Single-Walled Carbon Nanotubes: Sorting and Tuning the Electronic Properties.

We conducted the filling of single-walled carbon nanotubes (SWCNTs) with nickelocene molecules and separation of the filled SWCNTs by conductivity type by density-gradient ultracentrifugation. We tailored the electronic properties of nickelocene-filled purely metallic SWCNTs by thermal treatment in high vacuum. Our results demonstrated that annealing at low temperatures (360–600 °C) leads to n-doping of SWCNTs, whereas annealing at high temperatures (680–1200 °C) results in p-doping of SWCNTs. We found a correlation between the chemical state of the incorporated substances at different annealing temperatures and its influence on the electronic properties of SWCNTs.

Superconducting Scanning Tunneling Microscope Tip to Reveal Sub-millielectronvolt Magnetic Energy Variations on Surfaces.

Combining the complex ordering ability of molecules with their local magnetic properties is a little-explored technique to tailor spin structures on surfaces. However, revealing the molecular geometry can be demanding. Nickelocene (Nc) molecules present a large spin-flip excitation leading to clear changes of conductance at the excitation-threshold bias. Using a superconducting tip, we have the energy resolution to detect small shifts of the Nc spin-flip excitation thresholds, permitting us to reveal the different individual environments of Nc molecules in an ordered layer. This knowledge allows us to reveal the adsorption configuration of a complex molecular structure formed by Nc molecules in different orientations and positions. As a consequence, we infer that Nc layers present a strong noncollinear magnetic-moment arrangement.

Ni-Co@carbon nanosheet derived from nickelocene doped Co-BDC for efficient oxygen evolution reaction

In recent years, metal–organic framework (MOF) have become versatile precursors for preparation of electrocatalysts toward oxygen evolution reaction (OER). In this work, a novel and facile MOF-mediated synthesis strategy is demonstrated for constructing efficient OER catalysts. The nickelocene molecules were introduced into the channels of 2-D layered MOF (Co-BDC) to form the nickelocene@Co-BDC hybrids by vapor deposition method. The Ni-Co@carbon nanosheet was obtained by direct carbonization of nickelocene@Co-BDC. The obtained bimetallic electrocatalyst exhibits an unprecedentedly high OER activity with a small overpotential (243 mV at current density of 10 mA cm−2) and a low Tafel slop (66.97 mVdec-1) in 1.0 M KOH aqueous solution, which is better than the most reported non-noble metal based electrocatalysts. Density functional theory (DFT) studies reveal that the synergetic effect of Ni and Co active sites affords superior electrocatalytic properties, further indicating this binary metal electrocatalyst is a promising candidate for sustainable energy applications.

Spin dependent transmission of nickelocene-Cu contacts probed with shot noise

The current $I$ through nickelocene molecules and its noise are measured with a low temperature scanning tunneling microscope on a Cu(100) substrate. Density functional theory calculations and many-body modeling are used to analyze the data. During contact formation, two types of current evolution are observed, an abrupt jump to contact and a smooth transition. These data along with conductance spectra ($dI/dV$) recorded deep in the contact range are interpreted in terms of a transition from a spin-1 to a spin-1/2 state that is Kondo screened. Many-body calculations show that the smooth transition is also consistent with a renormalization of spin excitations of a spin-1 molecule by Kondo exchange coupling. The shot noise is significantly reduced compared to the Schottky value of $2eI$ but no influence of the Kondo effect or spin excitations are resolved. The noise can be described in the Landauer picture in terms of spin-polarized transmission of $\approx$35% through two degenerate $d_\pi$-orbitals of the Nickelocene molecule.

Single molecules sense spin

Surface Magnetism Imaging surface magnetism, and, in particular, spin excitations of adsorbed molecules or films, is challenging. Verlhac et al. demonstrate spin-sensing capability by using the magnetic exchange interaction between a surface sample and the spin-excited states of a nickelocene molecule attached to a scanning tunneling microscope tip. The spatial dependence of the exchange field at the atomic scale enabled imaging of magnetic corrugation with atomic-scale lateral resolution for iron atoms and small islands of cobalt atoms absorbed on nonmagnetic copper surfaces. Science , this issue p. [623][1] [1]: /lookup/doi/10.1126/science.aax8222

Atomic-scale spin sensing with a single molecule at the apex of a scanning tunneling microscope.

Recent advances in scanning probe techniques rely on the chemical functionalization of the probe-tip termination by a single molecule. The success of this approach opens the prospect of introducing spin sensitivity through functionalization by a magnetic molecule. We used a nickelocene-terminated tip (Nc-tip), which offered the possibility of producing spin excitations on the tip apex of a scanning tunneling microscope (STM). When the Nc-tip was 100 picometers away from point contact with a surface-supported object, magnetic effects could be probed through changes in the spin excitation spectrum of nickelocene. We used this detection scheme to simultaneously determine the exchange field and the spin polarization of iron atoms and cobalt films on a copper surface with atomic-scale resolution.

Detection of Spin-Vibration States in Single Magnetic Molecules.

The spin states of magnetic molecules have advantageous attributes as carriers of quantum information. However, spin-vibration coupling in molecules causes a decay of excited spin states and a loss of spin coherence. Here, we detect excitations of spin-vibration states in single nickelocene molecules on Ag(110) with a scanning tunneling microscope. By transferring a nickelocene to the tip, the joint spin-vibration states with an adsorbed nickelocene were measured. Chemical variations in magnetic molecules offer the opportunity to tune spin-vibration coupling for controlling the spin coherence.

Controlled spin switching in a metallocene molecular junction

The active control of a molecular spin represents one of the main challenges in molecular spintronics. Up to now spin manipulation has been achieved through the modification of the molecular structure either by chemical doping or by external stimuli. However, the spin of a molecule adsorbed on a surface depends primarily on the interaction between its localized orbitals and the electronic states of the substrate. Here we change the effective spin of a single molecule by modifying the molecule/metal interface in a controlled way using a low-temperature scanning tunneling microscope. A nickelocene molecule reversibly switches from a spin 1 to 1/2 when varying the electrode–electrode distance from tunnel to contact regime. This switching is experimentally evidenced by inelastic and elastic spin-flip mechanisms observed in reproducible conductance measurements and understood using first principle calculations. Our work demonstrates the active control over the spin state of single molecule devices through interface manipulation.

Nickelocene-Precursor-Facilitated Fast Growth of Graphene/h-BN Vertical Heterostructures and Its Applications in OLEDs.

The direct growth of high-quality, large-area, uniform, vertically stacked Gr/h-BN heterostructures is of vital importance for applications in electronics and optoelectronics. However, the main challenge lies in the catalytically inert nature of the hexagonal boron nitride (h-BN) substrates, which usually afford a rather low decomposition rate of carbon precursors, and thus relatively low growth rate of graphene. Herein, a nickelocene-precursor-facilitated route is developed for the fast growth of Gr/h-BN vertical heterostructures on Cu foils, which shows much improved synthesis efficiency (8-10 times faster) and crystalline quality of graphene (large single-crystalline domain up to ≈20 µm). The key advantage of our synthetic route is the utilization of nickel atoms that are decomposed from nickelocene molecules as the gaseous catalyst, which can decrease the energy barrier for graphene growth and facilitate the decomposition of carbon sources, according to our density functional theory calculations. The high-quality Gr/h-BN stacks are proved to be perfect anode/protecting layers for high-performance organic light-emitting diode devices. In this regard, this work offers a brand-new route for the fast growth of Gr/h-BN heterostructures with practical scalability and high crystalline quality, thus should propel its wide applications in transparent electrodes, high-performance electronic devices, and energy harvesting/transition directions.

Efficient Spin-Flip Excitation of a Nickelocene Molecule.

Inelastic electron tunneling spectroscopy (IETS) within the junction of a scanning tunneling microscope (STM) uses current-driven spin-flip excitations for an all-electrical characterization of the spin state of a single object. Usually decoupling layers between the single object, atom or molecule, and the supporting surface are needed to observe these excitations. Here we study the surface magnetism of a sandwich nickelocene molecule (Nc) adsorbed directly on Cu(100) by means of X-ray magnetic circular dichroism (XMCD) and density functional theory (DFT) calculations and show with IETS that it exhibits an exceptionally efficient spin-flip excitation. The molecule preserves its magnetic moment and magnetic anisotropy not only on Cu(100), but also in different metallic environments including the tip apex. By taking advantage of this robusteness, we are able to functionalize the microscope tip with a Nc, which can be employed as a portable source of inelastic excitations as exemplified by a double spin-flip excitation process.

Density Functional Study of Molecular Orbitals of Cobaltocene and Nickelocene Molecules

The electronic structure and geometry optimization of cobaltocene and nickelocene molecules are calculated using DFT/B3LYP with the basis set of 6-31G (d).The Eigen values, Eigen vector and population analysis of the molecules show that the first 12 molecular orbitals in cobaltocene and 14 in nickelocene have contribution from 2pz orbitals of carbon of (C5H5)- and 4s, 4p and 3d orbitals of cobalt and nickel respectively. We found that the extent of involvement of metal orbitals in the two cases is different. In cobaltocene the maximum involvement out of 4s and 4p orbital is in the order 4s>4pz>4py>4px and in 3d orbitals the order is 3dyz>3dxy>3dxz>3d2z>3dx2-y2. The involvement of corresponding orbital in nickelocene with respect to the 4s and 4p orbitals is in the order of 4py>4px>4s>4pz and in 3d orbitals the order is 3dyz>3dx2-y2>3dxy>3dxz>3dz2 molecules. The total involvement of 3d, 4s and 4p orbitals of metal and 2pz orbitals of the ten carbon atoms of both ligands of (C5H5)- in cobaltocene and nickelocene respectively are 40.2388 and 38.3776 hence we can conclude that cobaltocene is more stable than nickelocene. Similar results are found from calculation of parameters like dipole moment, HOMO-LUMO gap and Mullikan charge distribution. The population analysis shows that only 2pz orbitals of carbon of (C5H5)- and 3d orbitals of metal provide electrons to MOs of cobaltocene and nickelocene.

Quantum transport of the single metallocene molecule

The Quantum transport of three single metallocene molecule is investigated by performing theoretical calculations using the non-equilibrium Green's function method combined with density functional theory. We find that the three metallocen molecules structure become stretched along the transport direction, the distance between two Cp rings longer than the other theory and experiment results. The lager conductance is found in nickelocene molecule, the main transmission channel is the electron coupling between molecule and the electrodes is through the Ni dxz and dyz orbitals and the s, dxz, dyz of gold. This is also confirmed by the highest occupied molecular orbital resonance at Fermi level. In addition, negative differential resistance effect is found in the ferrocene, cobaltocene molecules, this is also closely related with the evolution of the transmission spectrum under applied bias.

Temperature-dependent inner tube growth and electronic structure of nickelocene-filled single-walled carbon nanotubes

Abstractauthoren We have filled single-walled carbon nanotubes (SWCNTs) with nickelocene and transformed them to double-walled carbon nanotubes by annealing in vacuum. We study the temperature-dependent inner tube growth by means of Raman spectroscopy and investigate the SWCNTs’ electronic structure by X-ray photoelectron spectroscopy. It is found that larger diameter inner tubes grow at higher temperatures than smaller diameter tubes. Both electron and hole doping of SWCNTs are achieved as the encapsulated nickelocene molecules are reacted at temperatures in the range from 250 up to 1200° C.

Doping of single-walled carbon nanotubes controlled via chemical transformation of encapsulated nickelocene.

Controlled doping of carbon nanotubes is elemental for their electronic applications. Here we report an approach to tune the polarity and degree of doping of single-walled carbon nanotubes via filling with nickelocene followed by encapsulated reactions. Using Raman, photoemission spectroscopy and transmission electron microscopy, we show that nickelocene molecules transform into nickel carbides, nickel and inner carbon nanotubes with reaction temperatures as low as 250 °C. The doping efficiency is determined for each chemical component. Synchronous charge transfer among the molecular components allows bipolar doping of the carbon nanotubes to be achieved in a broad range of ±0.0012 e(-) per carbon.

Heat capacity of paramagnetic nickelocene: Comparison with diamagnetic ferrocene

Nickelocene [bis(η5-cyclopentadienyl)nickel: Ni(C5H5)2, electron spin S=1, the ground state configuration 3A2g] is paramagnetic and belongs to a typical molecule-based magnet. Heat capacities of nickelocene have been measured at temperatures in the 3−320K range by adiabatic calorimetry. By comparing with those of diamagnetic ferrocene crystal, a small heat capacity peak centered at around 15K and a sluggish hump centered at around 135K were successfully separated. The low-temperature peak at 15K caused by the spin is well reproduced by the Schottky anomaly due to the uniaxial zero-field splitting of the spin S=1 with the uniaxial zero-field splitting parameter D/k=45K (k: the Boltzmann constant). The magnetic entropy 9.7JK−1mol−1 is substantially the same as the contribution from the spin-manifold Rln3=9.13JK−1mol−1 (R: the gas constant). The sluggish hump centered at around 135K arises from rotational disordering of the cyclopentadienyl rings of nickelocene molecule. The enthalpy and entropy gains due to this anomaly are 890Jmol−1 and 6.9JK−1mol−1, respectively. As the hump spreads over a wide temperature region, separation of the hump from the observed heat capacity curve involves a little bit ambiguity. Therefore, these values should be regarded as being reasonable but tentative. The present entropy gain is comparable with 5.5JK−1mol−1 for the sharp phase transition at 163.9K of ferrocene crystal. This fact implies that although the disordering of the rings likewise takes place in both nickelocene and ferrocene, it proceeds gradually in nickelocene and by way of a cooperative phase transition in ferrocene. A reason for this originates in loose molecular packing in nickelocene crystal. Molar heat capacity and the standard molar entropy of nickelocene are larger than those of ferrocene beyond the mass effect over the whole temperature region investigated. This fact provides with definite evidences for the loose molecular packing in nickelocene crystal.

Effects of the covalent linker groups on the spin transport properties of single nickelocene molecules attached to single-walled carbon nanotubes

The understanding of how the spin moment of a magnetic molecule transfers to a carbon nanotube, when the molecule is attached to it, is crucial for designing novel supramolecular spin devices. Here we explore such an issue by modeling the spin transport of a single-walled carbon nanotube grafted with one nickelocene molecule. In particular we investigate how the electron transport becomes spin-polarized depending on the specific linking group bonding nickelocene to the nanotube. We consider as linkers both aziridine and pyrrolidine rings and the amide group. Our calculations show that, at variance with aziridine, both pyrrolidine and amide, do alter the sp(2) character of the binding site of the nanotube and thus affect the transmission around the Fermi level. However, only aziridine allows transferring the spin polarization of the nickelocene to the nanotube, whose conductance at the Fermi level becomes spin-polarized. This suggests the superiority of aziridine as a linker for grafting magnetic molecules onto carbon nanotubes with efficient spin filtering functionality.