Electron-bombarded ⟨110⟩-oriented tungsten tips for stable tunneling electron emission

Designing 2D stripe winding network through crown-ether intermediate Ullmann coupling on Cu(111) surface

Chemical synthesis typically yields the most thermodynamically stable ordered arrangement, a principle also governing surface synthesis on an atomically level two-dimensional (2D) surface, fostering the creation of structured 2D formations. The linear connection arising from energetically stable chemical bonding precludes the generation of a 2D random network comprised of one-dimensional (1D) convoluted stripes through on-surface synthesis. Nonetheless, we underscored that on-surface synthesis possesses the capability not solely to fashion a 2D ordered linear network but also to fabricate a winding 2D network employing a precursor with a soft ring and intermediate state bonding within the Ullmann reaction. Here, on-surface synthesis was exhibited on Cu(111) employing a 2D self-assembled monolayer array of 4,4',5,5'-tetrabromodibenzo[18]crown-6 ether (BrCR) precursors. These precursors were purposefully structured, with a crown ether ring at the core and Br atoms positioned at the head and tail ends, facilitating preferential connections along the elongated axis to foster a 1D stripe configuration. We illustrate how adjustments in the quantities of the intermediate state, serving as a primary linkage, can yield a labyrinthine, convoluted winding 2D network of stripes. The progression of growth, underlying mechanisms, and electronic structures were scrutinized using an ultrahigh vacuum low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) setup combined with density functional theory (DFT) calculations. This experimental evidence opens a novel functionality in leveraging on-surface synthesis for the formation of a 2D random network. This discovery holds promise as a pioneering constituent in the construction of a ring host supramolecule, augmenting its capability to ensnare guest atoms, molecules, or ions.

Carbon Monoxide Stripe Motion Driven by Correlated Lateral Hopping in a 1.4 × 1.4 Monolayer Phase on Cu(111)

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.

Simulation-guided nanofabrication of high-quality practical tungsten probes

Micro/nanoscale tungsten probes are widely utilized in the fields of surface analysis, biological engineering, etc. amongst several others. This work performs comprehensive dynamic simulations on the influences of electric field distribution, surface tension and the bubbling situation on electrochemical etching behaviors, and then the tip dimension. Results show that the etching rate is reliant on the electric field distribution determined by the cathode dimension. The necking position lies in the meniscus rather than at the bottom of the meniscus. A bubble-free condition is mandatory to stabilize the distribution of OH⁻ and WO₄²⁻ ions for a smooth tungsten probe surface. Such simulation-guidance enables the nanofabrication of probes with a high aspect ratio (10 : 1), ultra-sharp tip apex (40 nm) and ultra-smooth surface. These probes have been successfully developed for high-performance application with Scanning Tunneling Microscopy (STM). The acquired decent atomic resolution images of epitaxial bilayer graphene robustly verify the feasibility of the practical level application of these nanoscale probes. Therefore, these nanoscale probes would be of great benefit to the development of advanced analytical science and nano-to-atomic scale experimental science and technology.

Dynamic simulation is employed to reveal the mechanism of electrochemical nanofabrication of nanoscale probes for atomic resolution imaging in STM.

Fabrication of tungsten tip probes within 3 s by using flame etching

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.

CO-tip manipulation using repulsive interactions

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.

Controlled Deposition Number of Organic Molecules Using Quartz Crystal Microbalance Evaluated by Scanning Tunneling Microscopy Single-Molecule-Counting

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 ( N_QCM) show good agreement with those counted from the STM images ( N_STM) within the error of ±25%. The results ensure the QCM capability for controlling the deposition number of organic molecules with high accuracy, that is, if one needs to deposit 100 molecules on the substrate, QCM control promises deposition of 100 ± 25 molecules.

Energy gap opening by crossing drop cast single-layer graphene nanoribbons

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 G_o) 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.

Room temperature stable film formation of π-conjugated organic molecules on 3d magnetic substrate

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.