Low-temperature Ultrahigh Vacuum Scanning Research Articles

Low noise current-to-voltage converter and vibration damping system for a low-temperature ultrahigh vacuum scanning tunneling microscope

The design of low-temperature tunnel current converters and the design of a low-temperature damping stage for a scanning tunneling microscope (STM) are presented. The current amplifiers are able to measure very low tunnel current (down to 0.25 pA), while preserving a sufficient bandwidth for topographic and spectroscopic measurements and with very low noise characteristics (down to 3 fA Hz−1/2). The design of a compact low-temperature magnetic damping stage with a resonance frequency of about 7 Hz, protecting the microscope mechanics against mechanical vibrations of the cryostat is also described. The damped stage with the microscope mechanics is in contact with the cryostat during cooling, while during the STM measurements, it is mechanically isolated.

A low-temperature ultrahigh vacuum scanning force microscope with a split-coil magnet

We present the design of a scanning force microscope, which is optimized for magnetic force microscopy experiments. It can be operated at temperatures down to 5.2 K, in ultrahigh vacuum, and in magnetic fields of up to 5 T. Cooling is provided by a liquid helium bath cryostat, and the magnetic field is generated by a superconducting split-coil magnet. The design allows easy access from the side through a shutter system for fast in situ tip and sample exchange, while the microscope stays at temperatures below 25 K. The microscope itself features an all-fiber interferometric detection system, a 7.5×7.5 μm2 scan area at 5.2 K, and an xy table. The topographic resolution is demonstrated by imaging monoatomic steps on a nickel oxide surface, while magnetic contrast is shown on cobalt platinum multilayers and on a manganite perovskite film.

Electron standing-wave observation in the Pd overlayer on Au(111) and Cu(111) surfaces by scanning tunneling microscopy

Electron standing waves in monolayer-high Pd overlayers grown on the Au(111) and the Cu(111) surfaces were observed by low-temperature ultrahigh vacuum scanning tunneling microscopy. We found that the period of the oscillation in the Pd overlayers is longer than that of the clean surface of both Au(111) and Cu(111) substrates. The longer wavelength on the Pd overlayers is explained by an upward shift in energy level of the two-dimensional surface states.

Small area, low-inductance niobium SQUID for the detection of single atomic spin-flips

We report progress towards a superconducting quantum interference device (SQUID) based system capable of detecting single atomic spin-flips. To date we have designed and had manufactured a niobium SQUID with a loop area of 3×10 −6 m by 3×10 −6 m. We present calculations which show that it is capable of detecting a few spins in a 1 Hz bandwidth, and experimental results are given which show that it operates close to the quantum-limit regime with the expected sensitivity. The SQUID is designed so that it will fit into a sample holder that will be loaded into a low-temperature ultra-high vacuum scanning tunnelling microscope (LT UHV STM). The STM will be used to image and manipulate trapped ad-atoms within the SQUID loop.

Atomic-Scale Insights into Hydrodesulfurization

We have used low-temperature ultrahigh vacuum scanning tunneling microscopy to gain atomic-scale insights into the hydrodesulfurization process. Investigations of Ni adsorbed on the basal plane of MoS2 revealed that the Ni adatoms are highly mobile due to their weak interaction with the exposed sulfur atoms. The Ni adatoms rapidly diffuse about the surface down to 77 K, and even at 4 K are easily manipulated with the STM probe tip. Spectroscopic imaging of Ni clusters adsorbed on the MoS2 basal plane reveals that their electronic structure is well suited to bind nucleophilic reactant species. From low-temperature STM images of thiophene adsorbed on Ni{110}, a bonding geometry is proposed. New roles for metal promoter atoms in hydrodesulfurization catalysis, based on these results, are suggested.

A low-temperature ultrahigh vacuum scanning tunneling microscope with a split-coil magnet and a rotary motion stepper motor for high spatial resolution studies of surface magnetism

We present the design of a new ultrahigh vacuum scanning tunneling microscope (STM) which operates at T<20 K inside the bore of a 2.5 T superconducting split-coil magnet. The tip/sample region can easily be controlled visually, thus allowing safe and fast exchange of samples and tips while the microscope stays at low temperatures. A newly developed rotary motion stepper motor is presented which allows rotation of the sample by >270° about an axis perpendicular to the tip axis. This feature allows metal or molecular beam evaporation normal to the sample surface. Even more important, by means of this device tip and sample can be brought into a parallel or antiparallel magnetic configuration thus opening a novel approach to the study of magnetic phenomena on an atomic length scale. In addition, measurements of the magneto-optical Kerr effect can be carried out without removing the sample from the STM. Also a new tip exchange mechanism is described. The microscopic and spectroscopic performance of the new instrument is illustrated on Au(111)/mica, on Tb(0001)/W(110), and on Gd(0001)/W(110).

TUNNELING ASYMMETRY IN SUPERCONDUCTING Nd1Ba2Cu3Oy SINGLE CRYSTALS

Clean, stable, and atomically flat (001) surfaces of the as-prepared Nd 1 Ba 2 Cu 3 O y (Nd123) single crystals have been investigated using a Low Temperature UltraHigh Vacuum Scanning Tunneling Microscope/Spectroscopy (STM/STS) system at 10 K. Several sets of highly reproducible STS data show the existence of an asymmetry in the tunneling spectra in superconducting Nd123 single crystals. The asymmetry is found to be intrinsic and can be explained by assuming that the pairing potential is a function of the kinetic energy of the charge carriers. The STS data are compared with the predictions of different theoretical models. The data on the (001) surfaces may be better fitted by assuming an S wave symmetry. Pure d(x2-y2) symmetry is ruled out.

Mobile Promoters on Anisotropic Catalysts: Nickel on MoS2

We have imaged the MoS2 basal plane bare and with adsorbed Ni atoms at 298, 77, and 4 K using a low-temperature ultrahigh vacuum scanning tunneling microscope. Ni atoms freely diffuse across the surface even at 77 K. At 4 K, isolated atoms and small clusters of Ni are stable and can be imaged. At 4 K, Ni atoms can be easily manipulated with the microscope tip. Spectroscopic measurements reveal that Ni adatoms create favorable electronic structures for binding nucleophilic reactants. In addition to this, our observations suggest two new roles for the Ni promoter atoms: (1) increasing the sticking probability of sulfur-containing hydrocarbons by binding them to the unreactive basal planes of MoS2 and (2) transporting these hydrocarbons as inorganic complexes to the active sites for reaction.

Local Electronic Properties in the Presence of Internal and External Magnetic Fields Studied by Variable-Temperature Scanning Tunneling Spectroscopy

We have developed a new low-temperature ultrahigh vacuum scanning tunneling microscope (UHV-STM) setup combined with a solenoid and a split-pair magnet. The STM can be operated in the 10-11 mbar range, down to 7 K and in a rotatable magnetic field of up to 7 T perpendicular and 2 T parallel to the sample surface. This LT-UHV-STM setup was applied to study the Landau level quantization in n-type InAs(110). In addition to the observation of the Landau level splitting of the conduction band we were able to determine the energy dependence of the effective electron mass due to the nonparabolicity of the conduction band. Furthermore, we have studied the scattering of electron waves at dopant atoms located several nanometers (1–20 nm) below the surface. In a second study we have applied variable-temperature UHV-STM to investigate the temperature-dependent exchange splitting of a dz2 -like surface state of Gd(0001) thin films. The experimental results indicate that the high spatial localization of the surface state helps to maintain a local exchange splitting even above the Curie temperature but does not result in an enhanced surface Curie temperature. Finally, we have demonstrated spin-polarized vacuum tunneling from a ferromagnetically-coated tip into the exchange-split surface state of Gd(0001).

A low-temperature ultrahigh-vacuum scanning tunneling microscope with rotatable magnetic field

We present a new design of a low-temperature ultrahigh-vacuum (UHV) scanning tunneling microscope setup with a combination of a solenoid and a split-pair magnet. The scanning tunneling microscope can be operated at temperatures down to 8 K and in a rotatable magnetic field of up to 1 T. Magnetic fields of up to 7 T perpendicular and 2 T parallel to the sample surface can be applied. The UHV part of the system allows in situ preparation and low energy electron diffraction/Auger analysis of samples. First topographic and spectroscopic measurements on p-InAs(110) are presented.

STM imaging and spectroscopy of single copperphthalocyanine molecules

A low-temperature ultrahigh-vacuum scanning tunneling microscope (STM) is used to image individual copperphthalocyanine (CuPc) molecules and to perform local spectroscopic measurements on these molecules. Evaporation of CuPc on liquid-nitrogen-cooled graphite substrates leads to nonclustered individual molecules at random positions. STM images at 35 K show protrusions with a height of about 0.2 nm and a diameter of about 1.0 nm, consistent with CuPc molecules that lie flat on the substrate. Current-voltage curves acquired with the STM tip positioned over CuPc show a pronounced peak in dI/dV vs V at a tip voltage of 0.4 ± 0.1 V. This is attributed to resonant tunneling through a molecular level of CuPc.

Nucleation, formation, and stability of benzene islands on

We have imaged stable islands of benzene molecules on the surface of at 77 K using a low temperature ultrahigh vacuum scanning tunneling microscope. The islands consist of a small number of benzene molecules and nucleate in straight lines on the surface. The ordering and stability of these clusters are discussed in terms of the molecular interactions with the substrate surface state electrons and intermolecular interactions.

Performance of the trial scanning atom probe: New approach to evaluate the microtip apex

In order to examine the feasibility of realizing the scanning atom probe, a trial instrument was constructed by modifying a low-temperature ultrahigh vacuum scanning tunneling microscope (STM). A scanning electron microscope was mounted on the trial instrument to inspect the relative position of both the open hole at the end of the funnel-shaped extraction electrode and a cusp apex on a specimen surface. In the preliminary experiment, a single sharp W tip of the STM and two extraction electrodes with respective open hole diameters of 10 and 20 μm were employed. The tip was brought near the electrode by employing a microscrew and a piezoactuator as with a conventional STM. The variation of the field emission current with the position of the center of the hole relative to the cusp apex was found to be significant enough to bring the apex precisely to the center of the hole. An interesting finding was that the field emission was observed from a tip with a radius of more than 2 μm at a relatively low voltage, indicating that the field confined to a narrow space between the electrode and the tip apex was high enough to induce field emission.

Tip-induced lifting of the Au{100} (hex)-phase reconstruction in a low temperature ultrahigh vacuum scanning tunneling microscope

The clean Au{100} surface is known to be reconstructed, forming a pseudohexagonal (5×27) outermost layer. This structure is observed both in ultrahigh vacuum (UHV) and in the electrochemical environment at potentials corresponding to small negative surface electronic charges. Using a UHV scanning tunneling microscope (STM) at 77 K we have observed that the reconstruction can be lifted at large positive sample biases. The 20% less dense bulk-terminated surface is produced and the excess material appears as irregularly shaped gold clusters. Over a period of a few minutes, however, the surface relaxes back to the pseudo-hexagonal phase, a process that can also be followed with the STM.

Atomic resolution in photon emission induced by a scanning tunneling microscope.

A low-temperature ultrahigh-vacuum scanning tunneling microscope (STM) is used to excite photon emission from Au(110) surfaces. In the detected photon intensity the (1 \ifmmode\times\else\texttimes\fi{} 2) reconstruction of the Au surface is clearly resolved. This first observation of atomic resolution in STM-induced photon emission is interpreted in terms of local variations of the electromagnetic interaction of tip and sample occurring at constant tunneling current. Similar effects are expected to affect other scanning probe methods, in particular those involving photons.

Scanning tunneling microscopy on W(100) at 80 K and room temperature

Using a low-temperature ultrahigh vacuum scanning tunneling microscope we have studied W(100) at 80 K and room temperature. For the low-temperature c(2 × 2) structure of clean W(100) we found a line pattern which is in agreement with the Debe-King model. The domain structure could clearly be resolved. The influence of H adsorption in the submonolayer coverage range could be identified by the appearance of local depressions with specific electronic properties and did not affect the c(2 × 2) structure. For high doses the H-saturated (1 × 1) structure could be resolved both at room and low temperature. At room temperature clean W(100) exhibited a weak line structure in the [100]-like directions with a separation of the W lattice parameter. Due to an inherent noise in the latter measurements (which could be explained by particular vibrational properties of the clean surface) the degree of order of the atomic positions in these lines could not be determined.

Low-temperature scanning tunneling microscopy

The novel technique of low-temperature scanning tunneling microscopy is used to study conventional and high- T c superconductors (spatial identification and distribution of the superconducting gap, vortex movement, etc.), but also custom-designed materials which are unstable at room temperature, e.g. xenon layers and size-selected clusters, or species whose rotational or vibrational movements have to be frozen in to achieve atomic resolution. We present the specific design and advantages of our low-temperature ultra-high vacuum scanning tunneling microscope along with recent results on the internal structure of C 60 fullerene molecules and their photon emitting properties.

A low-temperature ultrahigh vacuum scanning tunneling microscope system and tunneling spectra of the Bi2212 superconductor

We designed and constructed an ultrahigh vacuum scanning tunneling microscope (UHV-STM) which is available for the measurement of tunneling spectra at low temperature. The sample holder is cooled by thermal conduction to the stage that thermally connects with the refrigerator head. We cleaved Bi2Sr2CaCu2Oy (Bi2212 high Tc superconductor) single crystals under ultrahigh vacuum conditions, and measured tunneling spectra of the Bi–O surface at 35 K. The spectra were smeared due to the fact that the STM tip was at high temperature (≳100 K).

Spectroscopy of a single adsorbed atom.

We have used a low-temperature ultrahigh-vacuum scanning tunneling microscope to perform atomically localized spectroscopic measurements on single Fe atoms adsorbed onto the Pt(111) surface. Using a simple tunneling model, we are able to quantitatively deconvolute the tip and adatom local densities of states (LDOS) from the dI/dV spectra. We find that a resonance occurs in the adatom LDOS that is centered 0.5 eV above the Fermi energy. This feature has a width of approximately 0.6 eV, and occurs only when the tip is within angstroms (laterally) of the center of an Fe adatom

Field emission current-voltage curves as a diagnostic for scanning tunneling microscope tips

The current-voltage ( I–V) characteristics of a low-temperature ultrahigh-vacuum scanning tunneling microscope (STM) tip positioned >100 Å from a planar surface have been recorded. We find curvature in the Fowler-Nordheim plots (log 10( I/V 2) versus 1/ V) due to the tip-plane geometry as has been predicted theoretically. In addition, oscillations and sharp breaks in these I–V curves are observed over a wide voltage range, 50–1000 V. These I–V curves are used to characterize the STM tips prior to tunneling.