Understanding the origin of perpendicular magnetic anisotropy in surface-supported nanoclusters is crucial for fundamental research as well as data storage applications. Here, we investigate the perpendicular magnetic anisotropy energy (MAE) of bilayer cobalt islands on Au(111) substrate using spin-polarized scanning tunneling microscopy at 4.6 K and first-principles theoretical calculations. Au(111) substrate serves as an excellent model system to study the effect of nucleation site and stacking sequence on MAE. Our measurements reveal that the MAE of bilayer islands depends strongly on the crystallographic stacking of the two Co layers and nucleation of the third layer. Moreover, the MAE of Co atoms on Au(111) is enhanced by a factor of 1.75 as compared to that reported on Cu(111). Our first-principles calculations attribute this enhancement to the large spin–orbit coupling of the Au atoms. Our results highlight the strong impact of nanometer-scale structural changes in Co islands on MAE and emphasize the importance of spatially resolved measurements for the magnetic characterization of surface-supported nanostructures.Keywords: first-principles calculations; Magnetic anisotropy; magnetization switching; nanoclusters; scanning tunneling microscopy;

•Atom-specific interfacial electronic properties of the epitaxial graphene on Si-terminated SiC substrate were calculated.•Effect of van der Waals interaction correction for density functional theory (DFT) calculation were examined.•The band structure projected on the respective atomic orbitals of the C atoms in the buffer layer and uppermost Si atoms are calculated.•Demonstrated that the presence of the dangling bonds of the buffer layer carbon or uppermost Si atom in the substrate appears in scanning tunneling spectroscopy (STS) mapping image at certain voltages.We investigate the atom-specific interfacial electronic properties of the epitaxial graphene on Si-terminated SiC substrate using density functional theory (DFT) calculation with van der Waals interaction correction, focusing on the dependency of the local electronic state on the chemical environment. The band structure projected on the respective atomic orbitals of the carbon atoms in the buffer layer and uppermost Si atoms demonstrates that the dangling bonds of these atoms form band structures around the Fermi level. The contribution of each atom to the dangling bond states strongly depends on the chemical environment, i.e., the presence/absence of the interlayer Si-C covalent bond. This difference also affects the atom-specific local density of states of the top-layer graphene through its interaction with the substrate/buffer layer. We demonstrate that the bias voltage dependency of the scanning tunneling spectroscopy (STS) mapping image clearly reflects the presence of the dangling bonds of the buffer layer carbon or uppermost Si atom in the substrate, which would enable the detection of the buried dangling bond with an atomic spatial resolution via STS.

In this issue of ACS Nano, Nienhaus et al. report the optoelectronic properties of carbon nanotube chiral junctions with nanometer resolution in the presence of strong electric fields (∼1 V/nm). Here, we provide an overview of recent studies that combine scanning tunneling microscope (STM) and laser or microwave illumination. These techniques reveal nanoscale laser- or microwave-induced phenomena utilizing the intrinsic atomic resolution of the tunneling current, and do not require substantial modification of the STM itself. The merits of atomic-scale spatial resolution and chemical sensitivity of the laser or microwave spectroscopes make these techniques useful for nanoscale characterization.

We review recent studies of double-decker and triple-decker phthalocyanine (Pc) molecules adsorbed on surfaces in terms of the bonding configuration, electronic structure and spin state.The Pc molecule has been studied extensively in surface science. A Pc molecule can contain various metal atoms at the center, and the class of the molecule is called as metal phthalocyanine (MPc). If the center metal has a large radius, like as lanthanoid metals, it becomes difficult to incorporate the metal atom inside of the Pc ring. Pc ligands are placed so as to sandwich the metal atom, where the metal atom is placed out of the Pc plane. The molecule in this configuration is called as a multilayer-decker Pc molecule. After the finding that the double-decker Pc lanthanoid complex shows single-molecule magnet (SMM) behavior, it has attracted a large attention. This is partly due to a rising interest for the ‘molecular spintronics’, in which the freedoms of spin and charge of an electron are applied to the quantum process of information. SMMs represent a class of compounds in which a single molecule behaves as a magnet.The reported blocking temperature, below which a single SMM molecule works as an quantum magnet, has been increasing with the development in the molecular design and synthesis techniques of multiple-decker Pc complex. However, even the bulk properties of these molecules are promising for the use of electronic materials, the films of multi-decker Pc molecules is less studied than those for the MPc molecules.An intriguing structural property is expected for the multi-decker Pc molecules since the Pc planes are linked by metal atoms. This gives an additional degree of freedom to the rotational angle between the two Pc ligands, and they can make a wheel-like symmetric rotation. Due to a simple and well-defined structure of a multi-decker Pc complex, the molecule can be a model molecule for molecular machine studies.The multi-decker Pc molecules can provide interesting spin configuration. The center metal atom, including a lanthanoid metal of Tb, tends to be 3+ cation, while the Pc ligand to be 2− anion. This realizes two-spin system, in which spins from 4f electrons and π radical coexist. Though the spins of 4f orbitals of those molecules have been studied, the importance of the π radicals has been highlighted recently from the measurement of electronic conductance properties of these molecules.In this article, recent researches on multi-decker Pc molecules are reviewed. The manuscript is organized with groups of chapters as follows: (1) Film formation, (2) Spin of TbPc2 film and Kondo resonance observation, (3) Rotation of double-decker Pc complex and chemical modification for spin control, (4) Device formation using double-decker Pc complex.

•Reduction of clean CeO2(111) surface was examined by exposing it to H atom.•Atomically flat and wide-terrace CeO2(111)/Ru(0001) was prepared.•STM detected hydroxyl species (OH) formed after H dosing on CeO2(111).•H atoms hop on the surface until they form a OH trimer, stable configuration.•Hopping H atom reacts with OH to form H2O, which desorbs leaving an oxygen defect.Reduction of CeO2(111)/Ru(0001) surface by atomic hydrogen was investigated using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). We observed the formation of oxygen vacancy trimers and hydroxyl trimers on the stoichiometric CeO2(111) surface when it was exposed to atomic hydrogen at room temperature. The reaction of an impinging hydrogen atom with a surface oxygen atom yields a hydroxyl species, which diffuse on the surface until stabilized by the formation of OH trimers. The hydrogen atoms were located at atop sites of the oxygen atoms in the topmost surface layer. A reaction between the hopping hydrogen atom and the hydroxyl species yields a water molecule, which is desorbed from the surface leaving an oxygen defect. The oxygen vacancies were also observed as a trimer of vacancies. XPS measurements showed an increase of a reduced Ce and hydroxyl species with an amount of exposed hydrogen atoms. The former was estimated by measuring the ratio of Ce3 +/Ce4 + in the Ce 3d components. Our study shows the formation of hydroxyl trimer species in atomic scale upon atomic hydrogen exposure to CeO2(111) surface which could offer new catalytic activity.

We demonstrate that the lattice formation of an adsorbed molecule decouples the molecule–substrate interaction to change the Kondo resonance, which occurs due to interactions between conduction electrons and the molecule’s unpaired spin. The double-decker bis(phthalocyaninato)terbium(III) complex, which is single-molecule magnet and forms a Kondo resonance on a Au(111) surface through an unpaired π-radical spin, is studied using scanning tunneling microscopy/spectroscopy (STM/STS). In the STS spectra, an unusual sharp, strong peak (peak A) is found only for the molecule in a film. The peak position of peak A (εA) cyclically shifts by several hundred millivolts as the STS tip position shifts along the outer circle of the molecule, reflecting the tilting of the upper phthalocyanine (Pc) ligand from the flat-lying lower Pc ligand. The Kondo resonance, which is detected as a sharp peak at the Fermi level, also shows cyclic variations of the peak width and intensity. As εA approaches EF, the Kondo temperature (TK) increases. We propose a model that peak A originates from the singly occupied molecular orbital state whose energy is shifted by an unscreened final state effect due to a decrease in the molecule–substrate chemisorptive interaction. We further examine this model using density functional theory calculations, confirming a decreased molecule–substrate interaction for molecules in the film compared to that of isolated molecules. Further calculations of a tilted upper Pc ligand configuration show a site-dependent, cyclic variation of the molecule–substrate interaction within a molecule.Keywords: Kondo resonance; phthalocyanine; scanning tunneling microscopy; scanning tunneling spectroscopy; single-molecule magnet; unscreened final state

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.Keywords: chirality; Kondo resonance; phthalocyanine; scanning tunneling microscopy; scanning tunneling spectroscopy; single-molecule magnet

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 (YPc2) 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 YPc2 molecules, the spin of the molecule vanished as shown by our conductance measurements and corroborated by the results of density functional theory calculations.

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.

We studied triple-decker tris(phthalocyaninato)yttrium (Y2Pc3) molecules deposited on a Au(111) surface with a low-temperature scanning tunneling microscope (STM). It is shown that the triple-decker molecule can be successfully transferred to the Au(111) surface by a sublimation method in the ultrahigh vacuum condition. A monolayer film of Y2Pc3 is observed with a height of ∼0.55 nm from the bare Au(111) surface. The molecules are adsorbed with a flat-lying configuration and a pseudosquare lattice is formed. Inside each molecule, eight bright protrusions were observed in the occupied-state images, which correspond to the high density-of-state (DOS) area on both sides of four phenyl rings in the top phthalocyanine (Pc). Scanning tunneling spectroscopy (STS) data show distinct features both in occupied and in unoccupied states. The variation of STS spectra when the tip moved from the ligand position to the center of the molecule was small, showing a limited contribution from the center metal.

We show a 4.8 K STM observation of a double-decker bis(phthalocyaninato)yttrium (YPc2; Pc = phthalocyanine) molecule adsorption on Au(111) substrate. An eight-lobed structure was imaged as the submolecule STM contrast of a single molecule both in an isolated state and in a molecule film. This feature arises from the top Pc group, where both sides of the four phenyl rings are highlighted. As an isolated molecule, the adsorption orientation is determined by the lower Pc, the diagonal axis of which aligns parallel to the close-packed direction of Au(111). In a 2D film, a near-square molecule lattice appears with a unit of ∼1.47 × 1.38 nm2, and one of the lattice vectors is rotated by ∼15° from the close-packed direction. A tentative model is provided to illustrate the molecule array where neighboring molecules are rotated by ∼30° from each other. In this way, the lower Pcs should align along the [101̅] and [2̅11] directions (or their equivalent directions) alternately. All these facts illustrate the molecule−substrate and the molecule−molecule interactions in the initial adsorption and in the film accumulation.

Self-assemblies of a nonplanar dysprosium−phthalocyanine (DyPc) molecule on the reconstructed Au(111) substrate have been examined with a low-temperature scanning tunneling microscope (STM). A four-lobed structure with a dark center hole is imaged as an isolated DyPc molecule, where the Dy atom is expected to be positioned below the Pc plane and bound to the Au substrate. Careful measurements reveal that the axes of isolated DyPc molecules align well with the high symmetry directions of Au. This fact illustrates a strong molecule−substrate interaction. In a monolayer film, a square molecule lattice is observed, where the geometries of the molecules can be determined by our submolecularly resolved STM images. The deduced lattice vectors and the azimuthal angles of the molecules account for a dominant molecule−molecule interaction. In a bilayer growth regime, the bonding configurations of the molecules in the second layer coincide with that of the first layer. A similar azimuthal angle appearing in the two layers may indicate a columnar packing geometry of DyPc molecules.

The fabrication of Mn-based coordination networks on a Au(1 1 1) substrate with 4-4′-biphenyl dicarboxylic acid (BDA) as the linker molecule was investigated by scanning tunneling microscopy. Intriguing structures of ladder and rectangular-shaped networks were obtained by controlling the ratios of deposited amount of BDA molecules and Mn atoms. These structures are well explained by models in which BDA molecules occupy the perimeter of the rectangles and a pair of two Mn atoms are placed at the lattice points. For the rectangular structure, further two phases of a rectangular and a square networks were identified in which the paired Mn atoms were directing an identical direction and 90° rotated in an alternate manner, respectively. In addition, it was revealed that the open space surrounded by rectangle BDA molecules could capture a dimer of C60 molecules which were deposited on the Mn-based BDA networks.

We investigate the structure of submonolayer film of 4,4′-biphenyl dicarboxylic acid (BDA) molecules on Au(1 1 1)-22 × √3 reconstructed surface with the use of scanning tunneling microscopy (STM). The BDA molecules form ordered structures on Au(1 1 1) surface which are commensurate with the substrate. We have concluded that the molecule–molecule interaction is mainly through hydrogen bonding formed by a straight dimer of BDA molecules. The straight dimer can be expressed as 4s + 2t or its six crystallographic equivalents using the unit vectors of the gold substrate of s and t. The length of hydrogen bonding (O–H–O) is estimated to be 0.31 nm assuming nearest neighbor distance of gold atoms of 0.275 nm. The ordering shows a clear contrast with the case of BDA on Cu(1 0 0) surface [S. Stepanow, N. Lin, F. Vidal, A. Landa, M. Ruben, J.V. Barth, K. Kern, Nanoletters 5 (2005) 901] in which a square type of ordering of molecules is observed by the formation of hydrogen bonding between a carboxylate (COO) and a benzene ring. The clear difference of the ordered structure on Cu(1 0 0) and Au(1 1 1) surface demonstrates that the absence (presence) of deprotonation of carboxyl group of BDA molecule on Au(1 1 1) (Cu(1 0 0)) switches the straight and square type ordering of BDA molecules.

We have investigated the barrier energy for an ammonia molecule to penetrate into ice film by the use of infrared spectroscopy and Xe supersonic beam. After the ice film on a Pt(1 1 1) surface is exposed to ammonia molecules, an umbrella mode of ammonia molecules adsorbed on the ice film has been observed in infrared spectra. After the irradiation of accelerated Xe beam, we observed an energy shift of the mode of ammonia. The shifted mode is assigned to that of ammonia molecules at the interface between the ice film and the Pt(1 1 1) surface. This indicates that the collision with Xe beam induced the penetration of an ammonia molecule to the interface through the ice film. Using this feature, we estimate a barrier for penetration as 0.28 ± 0.03 eV which is much smaller than the one previously reported for bulk ice.

Recent studies of molecules on surface by the use of tunneling electrons of scanning tunneling microscopy (STM) are reviewed. Characteristic features of tunneling current of STM are used not only for real space imaging with an atomic resolution but also are utilized for chemical analysis of a single molecule and manipulation of a molecule with controlled excitation of their vibration modes. As promising candidates of chemical analysis at a single molecule level, inelastic tunneling spectroscopy (IETS) researches are discussed. The mechanism of vibration excitation in STM-IETS is compared with that in high-resolution electron energy loss spectroscopy (HR-EELS). Successful observations of vibration modes of molecules such as C–H stretching mode are introduced. At the same time, expected increase of the importance of resonant scattering mechanism in STM-IETS compared with the conventional IETS is examined, whose mechanism assumes the trapping of tunneling electrons in the adsorbate resonant state and the formation of temporary negative ions.Next, applications of tunneling electrons to the manipulations of adsorbates are discussed focusing on the phenomena induced by vibrational excitations. STM has a unique character of high current density, which cannot be obtained with conventional sources. The similarity between the vibration excitation with the high current of STM and the phenomenon known as desorption induced by multielectron transfer (DIMET) are discussed. Surface phenomena such as desorption, hopping, rotation and chemical reactions are excited as a consequence of sequential climbing of ladders of a vibrational mode formed in the potential well. The intrinsic high current of STM tunneling current enables such multiple excitations in a more controllable manner than the use of photogenerated hot electrons, and is expected to make a contribution to the understanding of elemental processes of surface phenomena.

We have investigated the bonding configuration of an isolated cis-2-butene molecule (CH3–CHCH–CH3) on the Pd(1 1 0) surface using a cryogenic STM at the sample temperature of 4.7 K. For the precise determination of the bonding site of the molecule, we utilize a novel method of rotating the molecule by injecting tunneling electrons into the molecule. We have observed the rotation of the molecule around the terminal site of a single Pd atom with hopping between four equivalent bonding sites when tunneling electrons with the energy of 170 meV were dosed. The bonding between the cis-2-molecule and the Pd(1 1 0) surface is considerably through π-bonding at the carbon double-bond, which makes bonding on the terminal site favorable. However, the analysis of the images of rotating molecule reveals that the CC is shifted from the terminal site and located at the off-symmetry position.

We review recent studies of double-decker and triple-decker phthalocyanine (Pc) molecules adsorbed on surfaces in terms of the bonding configuration, electronic structure and spin state.The Pc molecule has been studied extensively in surface science. A Pc molecule can contain various metal atoms at the center, and the class of the molecule is called as metal phthalocyanine (MPc). If the center metal has a large radius, like as lanthanoid metals, it becomes difficult to incorporate the metal atom inside of the Pc ring. Pc ligands are placed so as to sandwich the metal atom, where the metal atom is placed out of the Pc plane. The molecule in this configuration is called as a multilayer-decker Pc molecule. After the finding that the double-decker Pc lanthanoid complex shows single-molecule magnet (SMM) behavior, it has attracted a large attention. This is partly due to a rising interest for the ‘molecular spintronics’, in which the freedoms of spin and charge of an electron are applied to the quantum process of information. SMMs represent a class of compounds in which a single molecule behaves as a magnet.The reported blocking temperature, below which a single SMM molecule works as an quantum magnet, has been increasing with the development in the molecular design and synthesis techniques of multiple-decker Pc complex. However, even the bulk properties of these molecules are promising for the use of electronic materials, the films of multi-decker Pc molecules is less studied than those for the MPc molecules.An intriguing structural property is expected for the multi-decker Pc molecules since the Pc planes are linked by metal atoms. This gives an additional degree of freedom to the rotational angle between the two Pc ligands, and they can make a wheel-like symmetric rotation. Due to a simple and well-defined structure of a multi-decker Pc complex, the molecule can be a model molecule for molecular machine studies.The multi-decker Pc molecules can provide interesting spin configuration. The center metal atom, including a lanthanoid metal of Tb, tends to be 3+ cation, while the Pc ligand to be 2− anion. This realizes two-spin system, in which spins from 4f electrons and π radical coexist. Though the spins of 4f orbitals of those molecules have been studied, the importance of the π radicals has been highlighted recently from the measurement of electronic conductance properties of these molecules.In this article, recent researches on multi-decker Pc molecules are reviewed. The manuscript is organized with groups of chapters as follows: (1) Film formation, (2) Spin of TbPc2 film and Kondo resonance observation, (3) Rotation of double-decker Pc complex and chemical modification for spin control, (4) Device formation using double-decker Pc complex.

In this article, we investigate a single molecule magnet bis(phthalocyaninato)terbium(III) (TbPc2) molecule film by using low temperature STM. In order to investigate the effect of molecule–substrate interaction on the electronic and spin properties of the adsorbed molecule, we tune the molecule–substrate coupling by switching the substrate between Au(111) and Ag(111), the latter of which provides stronger interaction with the molecule than the former. Despite the enhanced chemical reactivity of the Ag(111) surface compared with Au(111), a well-organized pseudo-square film is formed. In addition, a checker-board type contrast variation is identified, which is well explained by the existence of two types of molecules whose rotational angle between the top and bottom Pc is θ = 45° (bright molecule) and θ = 30° (dark molecule). The expected stronger molecule–substrate interaction, however, appears as an intriguing dI/dV mapping image which reveals the spatial distribution of the density of states (DOS). We identify the contrast reversal in the dI/dV mapping for the molecules of θ = 45° and θ = 30° at the sample voltages of V = 0.7 eV and 1.1 eV. Combined with the density functional theory (DFT) calculation, we attribute this change to the shift of an electronic state due to the rotation of the mutual angle between the top and bottom Pc. For the spin behavior, we previously observed a Kondo resonance for the TbPc2 molecule adsorbed on the Au(111) surface. On the Ag(111) surface, the Kondo resonance is hardly observed, which is due to the annihilation of the π radical spin by the charge transfer from the substrate to the molecule. Instead we observe a Kondo peak for the molecule on the second layer, for which the spin recovers due to the reduction of the coupling with the substrate. In addition, when a magnetic field of 2 T normal to the surface is applied, the second layer molecule shows a sharp dip at the Fermi level. We attribute this to the inelastic tunneling feature caused by the spin flipping. This feature is not observed for the TbPc2/Au(111) system, suggesting that the decoupling between the TbPc2 molecule and Ag(111) by the presence of the first layer produces an inelastic feature in the tunneling spectra.