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On-surface metalation of metal-free π-conjugated planar molecules, such as metal-free tetraphenylporphyrin (2HTPP), using 3d transition metals prepared in ultrahigh vacuum (UHV), attracted signific...

Crown-ether (CR) ring molecules are known as host molecules for capturing guest species inside the ring. So far CR molecular films have only been grown by drop-casting a CR solution on an inert sub...

We report that the occurrence of a Kondo effect in magnetic molecules crucially depends on the point symmetry and oxidation state of the adsorbed species. Two different transition metal acetylacetonate (acac) compounds [$M{(\mathrm{acac})}_{3}$, with $M=\mathrm{Cr}(\mathrm{III})$ or Co(III)] adsorbed on a Cu(111) single crystal were investigated to demonstrate the interplay. After deposition, $\mathrm{Cr}{(\mathrm{acac})}_{3}$ molecules formed threefold symmetric $\mathrm{Cr}{(\mathrm{acac})}_{3}$ and twofold symmetric $\mathrm{Cr}{(\mathrm{acac})}_{2}$ by releasing a ligand, while $\mathrm{Co}{(\mathrm{acac})}_{3}$ molecules only formed twofold symmetric $\mathrm{Co}{(\mathrm{acac})}_{2}$. Threefold symmetric $\mathrm{Cr}{(\mathrm{acac})}_{3}$ molecules with a total electron spin $S=\frac{3}{2}$ exhibited no Kondo effect, while a clear Kondo resonance was observed in twofold $\mathrm{Cr}{(\mathrm{acac})}_{2}$ molecules. $\mathrm{Co}{(\mathrm{acac})}_{2}$ molecules surprisingly showed no Kondo resonance, even in the case of twofold symmetry, which is explained by a low-spin state of $S$ = 0. To analyze the results, a simple model is proposed based on the total electron spin and the symmetry of magnetic molecule. The present approach provides a feasible design strategy for single molecule magnets on metallic surfaces.

We report an ultra-high-vacuum low-temperature (4.6 K) scanning tunneling microscopy study of the molecular structure and dynamics of a carbon monoxide (CO) monolayer adsorbed at 20 K on Cu(111). We observe the well-known 1.4 × 1.4 phase of CO/Cu(111) for the first time in real-space imaging. At 4.6 K, the hexagonal symmetry of the monolayer is locally broken by the formation of stripes made of single and double CO rows of different apparent heights. Using density functional theory calculations, we assign the high rows to CO molecules adsorbed mostly at off-center top sites and the low rows to bridge sites. Groups of three or four very high molecules appear randomly and are assigned to nearest-neighbor, titled top site molecules. We observe simultaneous hopping of a few CO molecules between adjacent top and bridge sites, which produces the apparent motion of the stripe pattern.

Oxidative vaporization in air using a flame with a temperature range of 1950–2300 K was employed for controlling the tip shape made from a nipper-cut metal molybdenum (Mo) wire edge. An extremely high vapor pressure of MoO3 generated on the Mo surface in flame is a driving force behind the tip shape formation. Since the MoO3 vaporization rate follows the flame temperature gradient, we could control the tip apex shape by selecting the proper flame etching condition. The best condition to obtain a sharp tip apex based on statistical tests of dozens of Mo tips was obtained by Mo wire edge insertion into the 2100 K flame from the side for one second. This was repeated twice, which reproducibly provided a tip apex with a radius of 50–100 nm and a cone angle of 20–30 degrees. The present Mo tips, fabricated without aqueous solutions, were examined for their suitability as probe tips through air-scanning tunneling microscopy (STM), ultrahigh vacuum STM, field emission spectroscopy, and conductance measurements.

In this combined experimental and theoretical work, we report on the evolution of the skyrmion radius and its destruction in the system Co/Ru(0001) when an out-of-plane magnetic field is applied. At low fields, skyrmions are metastable and display an elliptical instability in which along the short axis, the spin texture approaches that of the spin-spiral phase and the long axis expands in order to go back to the spin-spiral ground state. At high fields, we observe round skyrmions of finite size up to the collapse field Bc, where they are destroyed and the topological charge is annihilated. We estimate Bc via numerical methods based on magnetization dynamics simulations parametrized by density functional theory calculations and compare it to experimental scanning tunneling microscopy observations obtained at ≈ 30 mK.

Precise control of organic molecule deposition on a substrate is quite important for fabricating single-molecule-based devices. In this study, we demonstrate whether a quartz-crystal microbalance (QCM) widely used for a film growth calibration has the ability to precisely measure the number of organic molecules adsorbed on a substrate. The well-known Sauerbrey’s equation is extended to formulate the relation between QCM resonant frequency shift and the number of adsorbed molecules onto the QCM surface. The formula is examined by QCM measurements of sublimation of π-conjugated organic molecules and direct counting of the deposited molecules one by one onto metal substrates, using ultrahigh vacuum low-temperature scanning tunneling microscopy (STM). It is revealed that the number of adsorbed molecules evaluated by QCM (NQCM) show good agreement with those counted from the STM images (NSTM) within the error of ±25%. The results ensure the QCM capability for controlling the deposition number of organic molecu...

Spin-polarized scanning tunneling microscopy (SP-STM) is a powerful tool to visualize spin-polarization vectors of sample surface atoms.

Band gap opening of a single-layer graphene nanoribbon (sGNR) sitting on another sGNR, fabricated by drop casting GNR solution on Au(111) substrate in air, was studied by means of scanning tunneling microscopy and spectroscopy in an ultra-high vacuum at 78 K and 300 K. GNRs with a width of ~45 nm were prepared by unzipping double-walled carbon nanotubes (diameter ~15 nm) using the ultrasonic method. In contrast to atomically-flat GNRs fabricated via the bottom-up process, the drop cast sGNRs were buckled on Au(111), i.e., some local points of the sGNR are in contact with the substrate (d ~ 0.5 nm), but other parts float (d ~ 1–3 nm), where d denotes the measured distance between the sGNR and the substrate. In spite of the fact that the nanoribbons were buckled, dI/dV maps confirmed that each buckled sGNR had a metallic character (~3.5 Go) with considerable uniform local density of states, comparable to a flat sGNR. However, when two sGNRs crossed each other, the crossed areas showed a band gap between −50 and +200 meV around the Fermi energy, i.e., the only upper sGNR electronic property changed from metallic to p-type semiconducting, which was not due to the bending, but the electronic interactions between the up and down sGNRs.

An important step toward molecule-based electronics is to realize a robust and well-ordered molecular network at room temperature. To this end, one key challenge is tuning the molecule–substrate electronic interactions that influence not only the molecular selfassembly but also the stability of the resulting structures. In this study, we investigate the film formation of π-conjugated metal-free phthalocyanine molecules on a 3d-bcc-Fe(001) whisker substrate at 300 K by using ultra-high-vacuum scanning tunneling microscopy. On bare Fe(001), hybridization between the molecular π and the Fe(001) d-states prevents the molecular assembly, resulting in the disordered patchy structures. The second- and third-layer molecules form densely packed films, while the morphologies show clear difference. The second-layer molecules partially form p(5 × 5)-ordered films with the rectangular edges aligned along the [100] and [010] directions, while the edges of the third-layer films are rounded. Remarkably, such film morphologies are stable even at 300 K. These findings suggest that the molecular self-assembly and the resulting morphologies in the second and third layers are affected by the substrate bcc(001), despite that the Fe-d states hybridize only with the first-layer molecules. The possible mechanism is discussed with the kinetic Monte Carlo simulation.

A tungsten (W) tip has been used as a standard tip probe because of its robustness at the highest boiling temperature; the use cases include a field emission (FE) electron source for scanning electron microscopy (SEM) and a scanning probe microscopy tip. The W tip probe has generally been fabricated through a chemical etching process with aqueous solutions. In this study, we propose a new method—flame etching. Without using aqueous solutions, a W tip probe was successfully fabricated within 3 s in air, which is very fast and convenient, and beneficial for mass production (additionally, no expensive setup is necessary). A W tip probe was obtained simply by putting a W wire into an oxygen-liquefied petroleum (O2+LP) gas flame (giving the highest temperature of ∼2300 K) through a microtorch for a few seconds. The obtained W tip provided atomically resolved scanning tunneling microscopic images. Also, since FE electrons were detected by applying ∼106 V/m, the tip can be used as an FE-SEM source. Generation and vaporization of WO3 on the W surface are important processes to form a tip shape.

One-nm-size single organic molecules have attracted interest as a key material to realize ultra-small spintronic devices such as single-molecule giant magnetoresistance memory. The origin of such magnetic phenomena is spin polarization vector in the single molecules induced by contacting magnetic electrodes. Atomically resolved spin-polarized STM has been used to measure quantitatively the spin polarization; however, in this study we point out that the reported method includes crucial ambiguity. By using the normalized differential conductance (dI/dV)/T, the true spin polarization of the single molecules on magnetic domains is revealed.

The realization of single molecular electronics is considered the next frontier to addressing and sustaining the storage needs of the future. In order to realize a single molecular device working at 300 K, two conditions must be satisfied: first, there must be no molecular diffusion, i.e., robust bonding between molecules and the contacting electrode, and second, stable electronic interface states. In this study, using a combination of 7-K and 300-K ultrahigh vacuum scanning tunneling microscopy/spectroscopy experiments and theoretical ab initio calculations, we investigated the adsorption of $\ensuremath{\pi}$-conjugated metal-free phthalocyanine (Pc) single molecules onto an Fe(001) whisker single crystal along with the resulting electronic interface structures. The Pc/Fe(001) system was found to prevent molecular diffusion even at 300 K, due to strong adsorption as well as the presence of a larger diffusion barrier than that of the Pc/Ag(001) system, in which molecules are known to diffuse at 300 K. The origin of such a robust bonding was studied by recovering the sample local density of states (LDOS) with the normalized $(dI/dV)/T$ curves, where the LDOS peaks are successfully explained by theoretical calculations.

A clean tungsten (W) tip apex with a robust atomic plane is required for producing a stable tunneling electron emission under strong electric fields. Because a tip apex fabricated from a wire by aqueous chemical etching is covered by impurity layers, heating treatment in ultra-high vacuum is experimentally known to be necessary. However, strong heating frequently melts the tip apex and causes unstable electron emissions. We investigated quantitatively the tip apex and found a useful method to prepare a tip with stable tunneling electron emissions by controlling electron-bombardment heating power. Careful characterizations of the tip structures were performed with combinations of using field emission I-V curves, scanning electron microscopy, X-ray diffraction (transmitted Debye-Scherrer and Laue) with micro-parabola capillary, field ion microscopy, and field emission microscopy. Tips were chemically etched from (1) polycrystalline W wires (grain size ∼1000 nm) and (2) long-time heated W wires (grain size larger than 1 mm). Heating by 10-40 W (10 s) was found to be good enough to remove oxide layers and produced stable electron emission; however, around 60 W (10 s) heating was threshold power to increase the tip radius, typically +10 ± 5 nm (onset of melting). Further, the grain size of ∼1000 nm was necessary to obtain a conical shape tip apex.

Understanding the interactions between a tip apex and a target atom or molecule is crucial for the manipulation of individual molecules with precise control by using scanning tunnelling microscopy (STM) and atomic force microscopy. Herein, we demonstrate the manipulation of target CO molecules on a Cu(111) substrate using a CO-functionalized W tip with atomic-scale accuracy. All experiments were performed in a home-built ultra-high vacuum STM system at 5 K. The CO-tip was fabricated by picking up a single CO molecule from a Cu(111) surface. In contrast to a metal tip, repulsive interactions occur between the CO-tip and the target CO molecule. This repulsive interaction promises perfect lateral hopping without any vertical hopping. Hopping events were directly monitored as sudden current drops in the simultaneously measured I-z curves. A larger barrier height between the CO-tip and the target CO (∼9.5 eV) was found from the slope of the I-z curve, which decreases the electron tunnelling probability between the tip and sample. Therefore, electron-driven manipulation cannot be a major trigger for the CO-CO repulsive manipulation. The CO-tip is able to manipulate only the target CO molecule, even when another CO molecule was located ∼0.5 nm away. Statistical measurements revealed that the nearest neighbour atop site is the energetically stable position after hopping. However, if the CO target has another CO molecule in a neighbouring position (denoted as a 'pair'), the target CO hops more than twice as far. This means that the CO-tip experiences a larger repulsive interaction from the pair. These observations of CO-tip manipulation are useful for the design of two-dimensional artificial molecular networks as well as for developing a better understanding of catalytic oxidation processes.

A simple method for fabricating single-layer graphene nanoribbons (sGNRs) from double-walled carbon nanotubes (DWNTs) was developed. A sonication treatment was employed to unzip the DWNTs by inducing defects in them through annealing at 500 °C. The unzipped DWNTs yielded double-layered GNRs (dGNRs). Further sonication allowed each dGNR to be unpeeled into two sGNRs. Purification performed using a high-speed centrifuge ensured that more than 99% of the formed GNRs were sGNRs. The changes induced in the electrical properties of the obtained sGNR by the absorption of nanoparticles of planar molecule, naphthalenediimide (NDI), were investigated. The shape of the I-V curve of the sGNRs varied with the number of NDI nanoparticles adsorbed. This was suggestive of the existence of a band gap at the narrow-necked part near the NDI-adsorbing area of the sGNRs.

A simple method for fabricating single-layer graphene nanoribbons (sGNRs) from double-walled carbon nanotubes (DWNTs) was developed. A sonication treatment was employed to unzip the DWNTs by inducing defects in them through annealing at 500 °C. The unzipped DWNTs yielded double-layered GNRs (dGNRs). Further sonication allowed each dGNR to be unpeeled into two sGNRs. Purification performed using a high-speed centrifuge ensured that more than 99% of the formed GNRs were sGNRs. The changes induced in the electrical properties of the obtained sGNR by the absorption of nanoparticles of planar molecule, naphthalenediimide (NDI), were investigated. The shape of the I-V curve of the sGNRs varied with the number of NDI nanoparticles adsorbed. This was suggestive of the existence of a band gap at the narrow-necked part near the NDI-adsorbing area of the sGNRs.

The control of atomically flat interfaces between iron (Fe) and insulating oxide films, such as the Fe/MgO(001) interface, is crucial for tunnel-magnetoresistance (TMR) devices. However, the realization of an ideal atomically flat and clean interface is rather difficult since iron easily binds to impurities such as oxygen. Atomic step defects and iron oxide at the interface could reduce TMR. In this study, the oxidization of an atomically flat and clean Fe(001)-whisker single crystal at different substrate and annealing temperatures was investigated with an ultrahigh-vacuum scanning tunneling microscope (STM). Annealing up to a temperature of 850 K was required to obtain ordered and atomically flat Fe(001)-p(1×1)O terraces after the oxidization with the coexistence of Fe–O nanoislands (∼1 nm in height, ∼50 nm in size). We found that the growth of such nanoislands, which enhances interface roughness, strongly depends on the substrate temperature (T S) during the oxidization. A T S lower than 300 K reduces the coverage by the nanoislands to less than 10%.