Temperature Ultrahigh Vacuum Scanning Tunneling Research Articles

Chemically imaging nanostructures formed by the covalent assembly of molecular building blocks on a surface with ultrahigh vacuum tip-enhanced Raman spectroscopy

Surface-bound reactions have become a viable method to develop nanoarchitectures through bottom-up assembly with near atomic precision. However, the bottom-up fabrication of nanostructures on surfaces requires careful consideration of the intrinsic properties of the precursors and substrate as well as the complex interplay of any interactions that arise in the heterogeneous two-dimensional (2D) system. Therefore, it becomes necessary to consider these systems with characterization methods sensitive to such properties with suitable spatial resolution. Here, low temperature ultrahigh vacuum scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS) were used to investigate the formation of 2D covalent networks via coupling reactions of tetra(4-bromophenyl)porphyrin (Br4TPP) molecules on a Ag(100) substrate. Through the combination of STM topographic imaging and TERS vibrational fingerprints, the conformation of molecular precursors on the substrate was understood. Following the thermally activated coupling reaction, STM and TERS imaging confirm the covalent nature of the 2D networks and suggest that the apparent disorder arises from molecular flexibility.

Room temperature molecular resolution nanopatterning of cyclopentene monolayers on Si(100) via feedback controlled lithography

Molecularly precise nanopatterning is demonstrated for a saturated organic monolayer on the Si(100) surface using room temperature ultra-high vacuum scanning tunneling microscopy. In particular, feedback controlled lithography enables the clean desorption of individual molecules from a highly-ordered cyclopentene monolayer at moderate negative sample bias, resulting in the exposure of isolated silicon dimers on an otherwise organically passivated surface. The quality and uniformity of the cyclopentene passivation layer is also quantified with X-ray photoelectron spectroscopy following exposure to ambient conditions, revealing that complete formation of the native oxide on silicon is suppressed for time scales exceeding 100 days.

Nanofabrication of Heteromolecular Organic Nanostructures on Epitaxial Graphene via Room Temperature Feedback-Controlled Lithography

Nanoscale control of surface chemistry holds promise for tailoring the electronic, optical, and chemical properties of graphene. Toward this end, the nanofabrication of sub-5-nm heteromolecular organic nanostructures is demonstrated on epitaxial graphene using room temperature ultrahigh vacuum scanning tunneling microscopy. In particular, monolayers of the organic semiconductor 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) are nanopatterned on epitaxial graphene using feedback-controlled lithography (FCL) and then used as chemical resists to template the deposition of N,N'-dioctyl-3,4,9,10-perylene-tetracarboxylic diimide (PTCDI-C8). The generality of this FCL-based nanofabrication procedure suggests its applicability to a wide range of fundamental studies and prototype device fabrication on chemically functionalized graphene.

Orthogonal Self-Assembly of Interconnected One-Dimensional Inorganic and Organic Nanostructures on the Si(100) Surface

Orthogonal, interconnected inorganic and organic one-dimensional nanostructures have been fabricated by parallel self-assembly on the Si(100) surface and investigated using room temperature ultrahigh vacuum scanning tunneling microscopy. In particular, bismuth nanowires were self-assembled on the clean Si(100)-2 x 1 surface perpendicular to the Si dimer rows, followed by hydrogen passivation of the surrounding Si surface. Styrene molecular chains were then self-assembled on the H-passivated Si(100)-2 x 1 surface to intersect perpendicularly with the Bi nanowires. This general approach can likely be applied to the wide range of inorganic and organic species that spontaneously form one-dimensional nanostructures on the Si(100) surface.

Cryogenic variable temperature ultrahigh vacuum scanning tunneling microscope for single molecule studies on silicon surfaces

The design and performance of a variable temperature ultrahigh vacuum (UHV) scanning tunneling microscope (STM) is presented. This STM operates from 8.2 to 300 K in a UHV environment with a base pressure of less than 6×10−11 Torr. Cooling is achieved from 300 to 80 K within 3.5 h and from 80 to 8.2 K within 4.5 h. The base temperature of 8.2 K is maintained at a liquid helium consumption rate of 0.9 l/h. This design allows for direct optical access to the tip-sample interface and direct line-of-sight dosing while the sample is mounted in the STM. The STM tip may be coarse translated laterally in two dimensions through a 6-mm-diam area at all temperatures. With the feedback loop off, the drift in the tip-sample spacing is approximately 0.008 Å/min at 8.2 K. Atomic resolution feedback controlled lithography is performed on hydrogen passivated Si(100) and differential tunneling conductance maps are gathered for isolated cyclopentene molecules on unpassivated Si(100), thus demonstrating that this system is well suited for studying single molecules on silicon surfaces from 8.2 to 300 K.

Cluster critical size effect during growth on a heterogeneous surface

Nucleation and growth on a complex surface containing a regular network of defects is investigated in the case of Co on Au(111). By means of variable temperature ultrahigh vacuum scanning tunneling microscopy it is shown that both monolayer and bilayer Co clusters can be found on the same surface reconstruction, depending on the nature of the particular nucleation site: two types of point dislocations located at the elbows of the herringbone reconstruction. The existence of a critical size for the structural transition from monolayer to the bilayer islands is inferred on the basis of experimental results, quenched molecular dynamics calculations, and Metropolis Monte Carlo simulations. The resulting phase diagram provides a detailed characterization of the different growth modes (monolayer and bilayer islands) as a function of temperature and coverage.

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.

In situ observation of gas-source molecular beam epitaxy of silicon and germanium on Si(001)

We have observed the development of the surfaces during gas-source growth of silicon and germanium in an elevated temperature ultrahigh vacuum scanning tunneling microscopy (STM), with near-atomic resolution under a range of temperature and flux, which are the two dominant parameters, and applied atomistic modeling to the structures seen by STM to enable us to give confident interpretation of the results. A key role in the growth of silicon and germanium on Si(001) from disilane and germane, respectively, is played by the surface hydrogen. The growth of germanium follows a similar path to that of silicon for the first few monolayers, after which the strain becomes relieved by periodic trenches, and eventually by a combination of faceted pits and clusters, both of which nucleate heterogeneously at surface defects. Understanding these processes is crucial to controlling the self-assembled Ge/Si quantum structures.

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.

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.