Performed Scanning Tunneling Microscopy Research Articles

STM observations of organometallic complexes on the TiO 2(1 1 0) and Si(1 1 1) surfaces

Gas-phase grafting of organometallic complexes is a relatively new technique for preparing supported metal catalysts. In order to understand the initial stage in this method, we have performed scanning tunneling microscopy (STM) observations of organo-gold and -iridium complexes adsorbed on TiO 2(1 1 0) and Si(1 1 1) surfaces before and after annealing in an ultra-high vacuum. The STM observations revealed that the surface structure was strongly dependent on the interaction between the surface and individual molecules, and the presence of oxygen atoms on the surface.

Reversible Switching of the Surface Conductance of Hydrogenated CVD Diamond Films

Chemical vapor deposited (CVD) diamond films have a controversial history regarding their surface electronic properties. Hydrogenation is known to induce a p-type conductive surface layer, which is not present on non-hydrogenated samples. The enhanced surface conductance can decrease significantly after annealing under high vacuum conditions at as low as 200 °C (a temperature which is sufficiently low to ensure that the hydrogen termination remains intact). Although the hydrogen is necessary for the surface conductance, the surface can be made poorly conductive without removing the hydrogen termination. We have performed scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) to better understand the origin of the enhanced surface conductance. Our STS experiments confirm that hydrogenation induces the appearance of a conductive surface layer, which can change considerably after high vacuum annealing. In this paper, we will discuss the conditions under which the surface conductivity can be restored. In particular we study the conductance changes during plasma hydrogenation and after exposing it to atmospheric conditions. Topographical STM scans confirm that the surface structure is not altered at low annealing temperatures.

Scanning electrochemical microscopy of a fuel-cell electrocatalyst deposited onto highly oriented pyrolytic graphite

The hydrogen evolution reaction (HER) has been examined on a platinum electrocatalysts (Johnson Matthey HSA platinum black) dispersed onto a flat highly oriented pyrolytic graphite (HOPG) electrode using an atomic force microscope (AFM) modified to perform scanning tunneling microscopy (STM) and scanning electrochemical microscopy (SECM). For both STM and SECM experiments the same Pt/Ir tips produced by electrochemical etching of Pt/Ir wire followed by coating with varnish have been used. The coating process leaves only the very end of the tip exposed. Positioning the SECM tip 42 nm from one of the particles allows monitoring of hydrogen evolution from that particle as a function of substrate potential. In a separate experiment the substrate has been polarized at a potential at which hydrogen evolution occurs and the SECM tip rastered over the surface to obtain images of the local concentration of hydrogen. This map indicates the activity of hydrogen production as a function of position.

Does the Macrostructure of Deoxyribonucleic Acid Molecules Adsorbed on Substrates by the Pulse Injection Method Reflect That in Solution?

In order to investigate whether the concentration-dependent macrostructure of Deoxyribonucleic acid (DNA) molecules can be retained after pulse injection, as compared to that originally present in the aqueous solution, we performed scanning tunneling microscopy (STM) imaging of plasmid DNA pulse-injected from an aqueous solution containing different DNA concentrations. The observed STM images revealed the presence of a relaxed circular macrostructure at low concentrations (1.2 µg/ml of DNA and 0.3 mM of Na+) whereas an entangled circular macrostructure was observed at high concentration (60 µg/ml of DNA and 15 mM of Na+). We conclude that plasmid DNA molecules retain the macrostructures possessed in the aqueous solution even after pulse injection onto a solid surface under vacuum environment.

Effect of the Dipole-Dipole Interaction on the Self-Assembly of Cyclodextrin Inclusion Complexes

We performed scanning tunneling microscopy (STM) observations on self-assembled α-cyclodextrin (CyD) inclusion complexes on HOPG for the guest compounds of (1) H2O, (2) MeOH (Methanol) and (3) p-NP (p-nitrophenol). The observed structures of α-CyD-H2O and α-MeOH complexes were channel-like with twofold symmetry, however, α-CyD-p-NP showed a structure different from that of the others, namely, a threefold symmetric arrangement. On the other hand, the dipole moments calculated for α-CyD-H2O and α-MeOH complexes were similar and directed from the secondary to the primary hydroxyl groups of the α-CyD. However, the dipole moment on the α-CyD-p-NP complex was oblique to the axis for the other two complexes. Taking into account this difference in the dipole moment and the hydrogen bonding between the CyDs, the structural differences observed by STM could be explained comprehensively. The obtained results indicate the existence of a high potential for the structural control of the self-assembled α-CyD complexes by changing the guest compounds.

Influence of electrostatic forces on the imaging process in scanning tunneling microscopy

We performed scanning tunneling microscopy (STM) experiments on layered semiconductor compound surfaces which suggest a significant influence of electrostatic forces (EF) on the imaging process. We performed STM experiments at varying tunneling biases on plain MoTe2 and ultrathin epitaxial WS2 films on MoTe2 substrates. We observed tunneling bias dependent height changes of up to several nanometers of layer edges and layer terraces. Both of the samples showed height increases of up to 8 nm for step edges depending on the tunneling bias. Only at certain biases the values known from X-ray diffraction for the layer thickness were approached. These observations cannot be explained solely by electronic effects and tunneling probability changes. Our evaluation of the results shows that mechanical changes of the morphology caused by varying EF interaction between tip and sample are likely to be the cause of these phenomena. In order to investigate the magnitude and influence of EF between tip and sample more closely we performed additional experiments with bias applied atomic force microscopy (BAAFM) which indicate a strong influence of EF on the imaging process. Atomic force curves with additionally applied bias yielded that the EF are in the range of several ten to several hundred nN depending on the tunneling bias. The corresponding charge density on tip and sample suggests the presence of single electrons and holes at the interface instead of homogeneous charge densities which might result in a pulsed EF interaction between tip and sample.

Design of a scanning tunneling microscope for in situ topographic and spectroscopic measurements within a commercial molecular beam epitaxy machine

A scanning tunneling microscope that performs scanning tunneling microscopy, scanning tunneling spectroscopy, and ballistic electron emission microscopy measurements on 2-in. wafers has been designed and constructed. The instrument is incorporated into the Si preparation chamber of a cryo-pumped Fisons V90H Si/III-V molecular beam epitaxy machine. Its design uses two commercial Burleigh inchworms: one for performing scanning tunneling microscopy measurements and a second for making a front contact which is necessary for the ballistic electron emission microscopy measurements. The substrate holder for the V90H system is designed to handle wafers up to 6 in. in diameter. Therefore, a custom 6-in.-diam. holder has been constructed which supports two 2-in. holders: one for performing reflection high energy electron diffraction measurements and a second which allows transfer of the wafer to the scanning tunneling microscope and also incorporates a removable shadow mask for growing metal/semiconductor Schottky diodes. Although the chamber vibrations generated by the cryopump and its compressor are quite severe, atomic-resolution images have been obtained with all of the system’s pumps in operation. The enhanced stability of our design is attributed to our unique support system of the inchworm and wafer which are both manufactured from machinable ceramic, the use of Be–Cu springs with an extension length of 16 in. for vibration isolation, and the use of silicone O-rings for vibration damping.

Controlled cluster condensation into preformed nanometer-sized pits

We have performed scanning tunneling microscopy (STM) in ultrahigh vacuum and transmission electron microscopy (TEM) on silver and gold clusters grown in preformed nanometer-sized pits on the surface of highly oriented pyrolytic graphite. We describe the preparation method and evaluate the three-dimensional shape of the clusters using a combination of STM and TEM applied to the same cluster sample. The nanometer-sized pits were essential to fix the clusters in position when using STM. The influence of the tip shape on the STM imaging of nanometer-sized clusters is discussed.

Internal structure of C60 fullerence molecules as revealed by low-temperature STM

We have performed Scanning Tunneling Microscopy (STM) in ultra-high vacuum at low temperatures (5 and 50K) of unordered and ordered C60 layers adsorbed on a Au(110) surface. STM topographs of the frozen C60 molecules reveal four symmetric patterns within single molecules, which may be associated with different orientations of the fullerenes on a highly corrugated gold substrate.

On the atomic structure and electronic properties of decapped GaAs(001)(2 × 4) surfaces

We have performed scanning tunneling microscopy and spectroscopy on the GaAs(001)(2 × 4) surface. The measurements were performed on arsenic capped samples grown ex-situ by MBE. The n-doped epilayers were grown on flat GaAs(001) n-doped wafers and substrates cut 2° towards (010). The epilayers were terminated with 100 Å of intrinsic material. Using STM we establish that capping does not compromise the surface quality of the (2 × 4) reconstruction. We find that provided the ex-situ MBE grown surfaces are of sufficient quality, the arsenic cap can be stripped off to yield surfaces equal to in-situ samples. Results from vicinal samples indicate that As from the cap adheres more strongly to step edges, which can be removed by subsequent annealing. The surfaces of all the samples studied were a mixture of (2 × 4) and c(2 × 8) domains. Tunneling I–V curves are interpreted as primarily due to tunneling out of the doubly occupied lone-pair states located on each surface As atom. Compared with GaAs(110), the spectra are much less sensitive to the distribution of empty states above E F . This behavior was independent of spatial location on the surface. The insensitivity to Ga derived empty states persists even when spectra are obtained above exposed Ga sites in the missing dimer rows where they may be expected to contribute. The measurements serve to locate the Fermi level on these samples pinned 0.6 ± 0.2 eV above the valence band maximum.

Scanning tunneling microscopy of the blue bronzes (Rb,K)0.3MoO3.

We have performed scanning tunneling microscopy of the blue bronzes ${\mathrm{K}}_{0.30}$${\mathrm{MoO}}_{3}$ and ${\mathrm{Rb}}_{0.30}$${\mathrm{MoO}}_{3}$ both above and below the charge-density-wave (CDW) transition temperature, ${\mathit{T}}_{\mathit{P}}$=180 K. We were able to image the Mo-O octahedra on the crystal surface at 295, 143, and 77 K. Below ${\mathit{T}}_{\mathit{P}}$, we were not able to image any additional superstructure due to the CDW, suggesting a relatively small CDW amplitude at the sample surface. Furthermore, the lattice images that we obtained below ${\mathit{T}}_{\mathit{P}}$ are unaffected by the sliding of the CDW.