Tadahiro Komeda

Observation and electric current control of a local spin in a single-molecule magnet.

In molecular spintronics, the spin state of a molecule may be switched on and off by changing the molecular structure. Here, we switch on and off the molecular spin of a double-decker bis(phthalocyaninato)terbium(III) complex (TbPc2) adsorbed on an Au(111) surface by applying an electric current via a scanning tunnelling microscope. The dI/dV curve of the tunnelling current recorded onto a TbPc2 molecule shows a Kondo peak, the origin of which is an unpaired spin of a π-orbital of a phthalocyaninato (Pc) ligand. By applying controlled current pulses, we could rotate the upper Pc ligand in TbPc2, leading to the disappearance and reappearance of the Kondo resonance. The rotation shifts the molecular frontier-orbital energies, quenching the π-electron spin. Reversible switching between two stable ligand orientations by applying a current pulse should make it possible to code information at the single-molecule level.

Direct Observation of Lanthanide(III)-Phthalocyanine Molecules on Au(111) by Using Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy and Thin-Film Field-Effect Transistor Properties of Tb(III)- and Dy(III)-Phthalocyanine Molecules

The crystal structures of double-decker single molecule magnets (SMM) LnPc(2) (Ln = Tb(III) and Dy(III); Pc = phthalocyanine) and non-SMM YPc(2) were determined by using X-ray diffraction analysis. The compounds are isomorphous to each other. The compounds have metal centers (M = Tb(3+), Dy(3+), and Y(3+)) sandwiched by two Pc ligands via eight isoindole-nitrogen atoms in a square-antiprism fashion. The twist angle between the two Pc ligands is 41.4 degrees. Scanning tunneling microscopy was used to investigate the compounds adsorbed on a Au(111) surface, deposited by using the thermal evaporation in ultrahigh vacuum. Both MPc(2) with eight lobes and MPc with four lobes, which has lost one Pc ligand, were observed. In the scanning tunneling spectroscopy images of TbPc molecules at 4.8 K, a Kondo peak with a Kondo temperature (T(K)) of approximately 250 K was observed near the Fermi level (V = 0 V). On the other hand, DyPc, YPc, and MPc(2) exhibited no Kondo peak. To understand the observed Kondo effect, the energy splitting of sublevels in a crystal field should be taken into consideration. As the next step in our studies on the SMM/Kondo effect in Tb-Pc derivatives, we investigated the electronic transport properties of Ln-Pc molecules as the active layer in top- and bottom-contact thin-film organic field effect transistor devices. Tb-Pc molecule devices exhibit p-type semiconducting properties with a hole mobility (mu(H)) of approximately 10(-4) cm(2) V(-1) s(-1). Interestingly, the Dy-Pc based devices exhibited ambipolar semiconducting properties with an electron mobility (mu(e)) of approximately 10(-5) and a mu(H) of approximately 10(-4) cm(2) V(-1) s(-1). This behavior has important implications for the electronic structure of the molecules.

Comparative Study of Amorphous and Crystalline (Ba, Sr)TiO3 Thin Films Deposited by Laser Ablation

(Ba0.5Sr0.5)TiO3 thin films (200-300 nm) were deposited on Pt-coated Si substrates by laser ablation at 500 and 650°C. The leakage currents of crystalline films grown at 650°C were found to be higher than those of amorphous films grown at 500°C. The crystalline thin films showed higher surface roughness than the amorphous films as measured by atomic force microscopy (AFM). A columnar grain structure was observed for crystalline films with a grain size of 20-30 nm by transmission electron microscope (TEM) analysis. These factors may be responsible for high leakage currents of crystalline films. Constant current injection measurements for Au/(Ba0.5Sr0.5)TiO3/Pt capacitors showed that electron trapping states near the top electrode interface were higher in number than at bottom electrode interface. This may be due to the presence of reactive sites on the surface of deposited films as observed by X-ray photoelectron spectroscopy (XPS) measurements.

Molecular Spintronics Based on Single‐Molecule Magnets Composed of Multiple‐Decker Phthalocyaninato Terbium(III) Complex

Unlike electronics, which is based on the freedom of the charge of an electron whose memory is volatile, spintronics is based on the freedom of the charge, spin, and orbital of an electron whose memory is non-volatile. Although in most GMR, TMR, and CMR systems, bulk or classical magnets that are composed of transition metals are used, this Focus Review considers the growing use of single-molecule magnets (SMMs) that are composed of multinuclear metal complexes and nanosized magnets, which exhibit slow magnetic-relaxation processes and quantum tunneling. Molecular spintronics, which combines spintronics and molecular electronics, is an emerging field of research. Using molecules is advantageous because their electronic and magnetic properties can be manipulated under specific conditions. Herein, recent developments in [LnPc]-based multiple-decker SMMs on surfaces for molecular spintronic devices are presented. First, we discuss the strategies for preparing single-molecular-memory devices by using SMMs. Next, we focus on the switching of the Kondo signal of [LnPc]-based multiple-decker SMMs that are adsorbed onto surfaces, their characterization by using STM and STS, and the relationship between the molecular structure, the electronic structure, and the Kondo resonance of [TbPc(2)]. Finally, the field-effect-transistor (FET) properties of surface-adsorbed [LnPc(2)] and [Ln(2)Pc(3)] cast films are reported, which is the first step towards controlling SMMs through their spins for applications in single-molecular memory and spintronics devices.

Local chemical reaction of benzene on Cu(110) via STM-induced excitation

We have investigated the mechanism of the chemical reaction of the benzene molecule adsorbed on Cu(110) surface induced by the injection of tunneling electrons using scanning tunneling microscopy (STM). With the dosing of tunneling electrons of the energy 2-5 eV from the STM tip to the molecule, we have detected the increase of the height of the benzene molecule by 40% in the STM image and the appearance of the vibration feature of the nu(C-H) mode in the inelastic tunneling spectroscopy (IETS) spectrum. It can be understood with a model in which the dissociation of C-H bonds occurs in a benzene molecule that induces a bonding geometry change from flat-lying to up-right configuration, which follows the story of the report of Lauhon and Ho on the STM-induced change of benzene on the Cu(100) surface. [L. J. Lauhon and W. Ho, J. Phys. Chem. A 104, 2463 (2000)]. The reaction probability shows a sharp rise at the sample bias voltage at 2.4 V, which saturates at 3.0 V, which is followed by another sharp rise at the voltage of 4.3 V. No increase of the reaction yield is observed for the negative sample voltage up to 5 eV. In the case of a fully deuterated benzene molecule, it shows the onset at the same energy of 2.4 eV, but the reaction probability is 10(3) smaller than the case of the normal benzene molecule. We propose a model in which the dehydrogenation of the benzene molecule is induced by the formation of the temporal negative ion due to the trapping of the electrons at the unoccupied resonant states formed by the pi orbitals. The existence of the resonant level close to the Fermi level ( approximately 2.4 eV) and multiple levels in less than approximately 5 eV from the Fermi level, indicates a fairly strong interaction of the Cu-pi(*) state of the benzene molecule. We estimated that the large isotope effect of approximately 10(3) can be accounted for with the Menzel-Gomer-Redhead (MGR) model with an assumption of a shallow potential curve for the excited state.

Geometrical characterization of pyrimidine base molecules adsorbed on Cu(1 1 0) surfaces: XPS and NEXAFS studies

The structures of pyrimidine base molecules of thymine and cytosine adsorbed onto Cu(110) surfaces (0 > 1.0 ML) have been discussed based on the results of X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The NEXAFS analysis for azimuthal orientation of molecules on surfaces indicates that the adsorbed molecules are in an upright configuration with their n plane parallel to the row structure ([110] direction), well below saturation coverage (θ ∼ 0.2 ML). Based on the chemical shifts observed in XP spectra (N1s, O1s region), the molecules have been determined to interact with surfaces through their nitrogen atom (lone-pair) next to the carbonyl group (C=O).

Variation of Kondo Peak Observed in the Assembly of Heteroleptic 2,3-Naphthalocyaninato Phthalocyaninato Tb(III) Double-Decker Complex on Au(111)

By using scanning tunneling microscopy (STM), we studied the heteroleptic double-decker complex TbNPcPc (NPc = naphthalocyaninato and Pc = phthalocyaninato), where two different planar ligands sandwich a Tb(III) ion and an unpaired π electron causes Kondo resonance upon adsorption on the Au(111) surface. Kondo resonance is a good conductance control mechanism originating from interactions between conduction electrons and a localized spin. Two types of adsorption geometries appear depending on which side contacts the substrate surface, which we call Pc-up and NPc-up molecules. They make intriguing molecular assemblies by segregation. In addition, different adsorption geometries and molecular assemblies provide a variety of spin and electronic configurations. Pc-up and NPc-up molecules both showed the Kondo resonance when they were isolated from other molecules, but their Kondo temperatures were different. A one-dimensional chain composed of only NPc-up molecules was found, in which the dI/dV plot showed a conversion from the Kondo peak to a dip at the Fermi energy. In addition, a two-dimensional lattice with an ordering of Pc-up and NPc-up molecules in an alternative manner was observed, in which no Kondo peak was detected in the molecule. The absence of the Kondo peak was accounted for by the change of azimuthal rotational angle of the two ligands of both molecules. The results imply that a molecule design and adsorption configuration tailoring can be used for the spin-mediated control of the electronic conductance of the molecule.

First Observation of a Kondo Resonance for a Stable Neutral Pure Organic Radical, 1,3,5-Triphenyl-6-oxoverdazyl, Adsorbed on the Au(111) Surface

We investigated spin states of stable neutral pure-organic radical molecules of 1,3,5-triphenyl-6-oxoverdazyl (TOV) and 1,3,5-triphenyl-6-thioxoverdazly (TTV) adsorbed on an Au(111) surface, which appears as a Kondo resonance because of spin-electron interaction. By using scanning tunneling spectroscopy (STS), a clear Kondo resonance was detected for the TOV molecule. However, no Kondo resonance was detected for TOV molecules with protrusions in the occupied state image and for TTV molecules. Spin-resolved DFT calculations showed that an unpaired π electron was delocalized over the adsorbed TOV molecule, which was the origin of the Kondo resonance. For the TOV molecules with protrusions, we proposed a model in which an additional H atom was attached to the TOV molecule. Calculations showed that, upon transfer of an electron to the verdazyl ring, the unpaired π electron disappeared, accounting for the absence of a Kondo resonance in the STS spectra. The absence of a Kondo resonance for the TTV molecule can be explained in a similar manner. In other words, electron transfer to the verdazyl ring occurs because of Au-S bond formation.

Epitaxial growth of CeO2(111) film on Ru(0001): scanning tunneling microscopy (STM) and x-ray photoemission spectroscopy (XPS) study.

We formed an epitaxial film of CeO2(111) by sublimating Ce atoms on Ru(0001) surface kept at elevated temperature in an oxygen ambient. X-ray photoemission spectroscopy measurement revealed a decrease of Ce(4+)/Ce(3+) ratio in a small temperature window of the growth temperature between 1070 and 1096 K, which corresponds to the reduction of the CeO2(111). Scanning tunneling microscope image showed that a film with a wide terrace and a sharp step edge was obtained when the film was grown at the temperatures close to the reduction temperature, and the terrace width observed on the sample grown at 1060 K was more than twice of that grown at 1040 K. On the surface grown above the reduction temperature, the surface with a wide terrace and a sharp step was confirmed, but small dots were also seen in the terrace part, which are considerably Ce atoms adsorbed at the oxygen vacancies on the reduced surface. This experiment demonstrated that it is required to use the substrate temperature close to the reduction temperature to obtain CeO2(111) with wide terrace width and sharp step edges.

Spin doping of individual molecules by using single-atom manipulation.

Being able to control the spin of magnetic molecules at the single-molecule level will make it possible to develop new spin-based nanotechnologies. Gate-field effects and electron and photon excitations have been used to achieve spin switching in molecules. Here, we show that atomic doping of molecules can be used to change the molecular spin. Furthermore, a scanning tunneling microscope was used to place or remove the atomic dopant on the molecule, allowing us to change the molecular spin in a controlled way. Bis(phthalocyaninato)yttrium (YPc(2)) molecules deposited on an Au (111) surface keep their spin-1/2 magnetic moment due to the small molecule-substrate interaction. However, when Cs atoms were carefully placed onto YPc(2) molecules, the spin of the molecule vanished as shown by our conductance measurements and corroborated by the results of density functional theory calculations.