Quasi-periodic excitation of the tunneling junction in a scanning tunneling microscope, by a mode-locked ultrafast laser, superimposes a regular sequence of 15 fs pulses on the DC tunneling current. In the frequency domain, this is a frequency comb with harmonics at integer multiples of the laser pulse repetition frequency. With a gold sample the 200th harmonic at 14.85 GHz has a signal-to-noise ratio of 25 dB, and the power at each harmonic varies inversely with the square of the frequency. Now we report the first measurements with a semiconductor where the laser photon energy must be less than the bandgap energy of the semiconductor; the microwave frequency comb must be measured within 200 μm of the tunneling junction; and the microwave power is 25 dB below that with a metal sample and falls off more rapidly at the higher harmonics. Our results suggest that the measured attenuation of the microwave harmonics is sensitive to the semiconductor spreading resistance within 1 nm of the tunneling junction. This approach may enable sub-nanometer carrier profiling of semiconductors without requiring the diamond nanoprobes in scanning spreading resistance microscopy.

The possible use of spin and magnets in place of charge and capacitors, respectively, to store and process information is well known. Magnetic tunnel junctions are being widely investigated and developed for magnetic random access memories. These are two terminal devices that change their resistance based on switchable magnetization of magnetic materials. They utilize the interaction between electron spin and magnets to read information from the magnets and write onto them. Such advances in memory devices could also translate into a new class of logic devices that offer the advantage of nonvolatile and reconfigurable information processing over transistors. Logic devices having a transistor-like gain and directionality could be used to build integrated circuits without the need for transistor-based amplifiers and clocks at every stage. We review device characteristics and basic logic gates that compute with spins and magnets from the mesoscopic to the atomic scale, as well as materials, integration, and fabrication challenges and methods.

The self-assembly of α,ω-dihexylsexithiophene molecules on an Au(111) surface was examined by using scanning tunneling microscopy at room temperature, revealing the internal molecular structures of the sexithiophene backbones and the hexyl side chains. The α,ω-dihexylsexithiophene formed a large and well-ordered monolayer in which the molecule lay flatly on the Au(111) surface and was separated into two chiral domains. A detailed observation reveals that the admolecules were packed in one lamellae with their molecular axis aligned along the main axis of the Au(111) substrate with their hexyl chains deviated from ⟨110⟩ direction of the Au(111) substrate by 12 ± 0.5°. In contrast to the behavior in the three-dimensional bulk structure, flat-lying adsorption introduced molecular chirality: right- and left-handed molecules separate into domains of two different orientations, which are mirror symmetric with respect to the ⟨121⟩ direction of the Au(111) substrate. Details of the adlayer structure and the chiral self-assembly were discussed here.

A scheme of photoabsorption spectroscopy based on scanning tunneling microscopy (STM) has been developed by using a supercontinuum light as the wideband light source of a Fourier transform interferometer for spectroscopic measurements. The performance was demonstrated for a sample of GaAs. The proof-of-concept test showed that the use of the supercontinuum light instead of halogen lamps greatly enhances the signal-to-noise ratio due to the high brilliance of the supercontinuum light emitted from a small core of the photonic crystal fiber that enables tight focusing of the spectroscopy light onto the sample beneath the STM tip.

We use a piezoelectric quartz tuning fork to calibrate the displacement of ceramic piezoelectric scanners that are widely employed in scanning probe microscopy. We measure the static piezoelectric response of a quartz tuning fork and find it to be highly linear, nonhysteretic and with negligible creep. These performance characteristics, close to those of an ideal transducer, make quartz transducers superior to ceramic piezoelectric actuators. Furthermore, quartz actuators in the form of a tuning fork have the advantage of yielding static displacements comparable to those of local probe microscope scanners. We use the static displacement of a quartz tuning fork as a reference to calibrate the three axis displacement of a ceramic piezoelectric scanner. Although this calibration technique is a nontraceable method, it can be more versatile than using calibration grids because it enables characterization of the linear and nonlinear response of a piezoelectric scanner in a broad range of displacements, spanning from a fraction of a nanometer to hundreds of nanometers. In addition, the creep and the speed dependent piezoelectric response of ceramic scanners can be studied in detail.

With exceptional carrier mobilities, mechanical strength, and optical transparency, graphene is a leading material for next-generation electronic devices. However, for most applications, graphene will need to be integrated with other materials, which motivates efforts to understand and tune its surface chemistry. In particular, the modification of graphene via organic functionalization holds promise for tuning the electronic properties of graphene, controlling interfaces with other materials, and tailoring surface chemical reactivity. Toward these ends, this article reviews recent work from our laboratory on noncovalent and covalent organic functionalization of graphene. Using ultrahigh vacuum scanning tunneling microscopy (UHV STM), the molecular ordering and electronic properties of organic adlayers on graphene are characterized at the molecular scale. In addition, UHV STM is employed to nanopattern these organic layers with sub-5 nm resolution, thus providing a pathway for producing graphene-based heteromolecular nanostructures.

Gd silicide nanostructures epitaxially grown on Si(001) are studied by plan-view transmission electron microscopy and associated nanobeam electron diffraction, as well as scanning tunneling microscopy. The nanobeam diffraction measurements show a direct correlation between the nanostructure morphology, either nanowires or islands, and the silicide crystal structure. Scanning tunneling microscopy shows a phase transformation from nanowires to islands that nucleate at nanowire intersections. A specific mechanism for this transformation is proposed that explains nanowire growth behavior previously observed on vicinal Si surfaces.

This brief article introduces the July 2004 issue of MRS Bulletin, focusing on Scanning Probe Microscopy in Materials Science.Those application areas of scanning probe microscopy (SPM) in which the most impact has been made in recent years are covered in the articles in this theme.They include polymers and semiconductors, where scanning force microscopy is now virtually a standard characterization method; magnetism, where magnetic force microscopy has served both as a routine analytical approach and a method for fundamental studies;tribology, where friction force microscopy has opened entirely new vistas of investigation;biological materials, where atomic force microscopy in an aqueous environment allows biosystems to be imaged and measured in a native (or near-native) state;and nanostructured materials, where SPM has often been the only approach capable of elucidating nanostructures.

Scanning tunneling microscopy and noncontact atomic force microscopy have been used to observe germanium growth on Si(001) and Si(111). The atomically resolved images provide invaluable information on heteroepitaxial film growth from the viewpoints of both industrial application and basic science. We briefly review the history of characterizing heteroepitaxial elemental semiconductor systems by means of scanning probe microscopy (SPM), where the Stranski–Krastanov growth mode can be observed on the atomic scale:the detailed phase transition from layer-by-layer growth to three-dimensional cluster growth was elucidated by the use of SPM. In addition, we comment on the potential of SPM for examining the spectroscopic aspects of heteroepitaxial film growth, through the use of SPM tips with well-defined facets.

The imaging and characterization of individual molecules and atomically thin, effectively one-dimensional crystals of rock salt and other halides encapsulated within single-walled carbon nanotubes are reviewed in this article. These species were imaged by conventional and super-resolved high-resolution transmission electron microscopy and by scanning tunneling microscopy, revealing the detailed atomic structure of these nanoscopic species.

Elevated temperature scanning tunneling microscopy is used to study oxides that are room temperature insulators but become sufficiently electrically conducting at higher temperatures to allow imaging to be performed. Atomic resolution images of NiO, CoO, and UO2 have been obtained in this fashion which allow surface structure and defect determination. To complement the experiments, modeling of the electronic surface structure reveals which atomic sites give rise to the contrast observed in the images. Low voltage scanning electron microscopy is used to image small equilibrium pores in UO2 single crystals to evaluate the surface energy ratio of the (111) to (001) surfaces.