Combined electric field and near-field scanning optical microscopy: modification of silicon surfaces

Received: 3 July 1998     

Delineation of a p–n junction using dC/dX imagery produced by shear-mode scanning capacitance microscopy

Using a laterally oscillating all-metallic probe, a scanning capacitance microscope (SCM) has been used to yield an image of the spatial derivative of the local capacitance, dC/dX, where C and X are the local capacitance and the axis of the probe tip locus on the sample surface, respectively. Bias fields, except for the ultra-high-frequency fields used for sensing the capacitance, are not necessary to detect the dC/dX signal, which yields an image delineating clearly the depletion region due to the p–n junction. Simultaneously with the dC/dX image, the new SCM can give images of topography and dC/dV if an alternating field V is applied between the probe and sample. Received: 19 March 2001 / Accepted: 22 March 2001 / Published online: 27 June 2001     

Scanning near-field optical microscopy

Scanning Near-field Optical Microscopy (SNOM) allows the investigation of optical properties on subwavelength scales. During the past few years, more and more attention has been given to this technique that shows enormous potential for imaging, sensing and modification at near-molecular resolution. This article describes the technique and reviews recent progress in the field.     

Optical oscillation modes of plasmon particles observed in direct space by phase-contrast near-field microscopy

Light distributions near resonant metal nanoparticles are recorded by a scattering-type scanning near-field optical microscope (s-SNOM), for the first time with a sub-particle-size resolution (<10 nm) and with simultaneous amplitude and phase contrast. The images depict the optical oscillation patterns of single plasmon particles. Examples are presented of particles excited in dominantly dipolar and quadrupolar modes, and also of closely spaced particles sustaining a gap mode. The gap mode can provide enhanced optical fields in nanometric spots for non-linear and single-molecule spectroscopy applications. Received: 20 June 2001 / Revised version: 3 August 2001 / Published online: 19 September 2001     

Scanning tunneling microscopy on self-assembled calix[4]resorcinarene monolayer adsorbates on Au(111)

Self-assembled Monolayers of calix[4]resorcinarene receptor molecules on Au(111) were studied by UHV scanning tunneling microscopy and X-ray photoelectron spectroscopy. Highly ordered monolayers were observed with domains oriented at an angle of 60° relative to each other. Molecularly resolved images were investigated and lattice constants found which depended on the preparation solvent. The STM images of two samples, one prepared in 1 mM chloroform/ethyl alcohol adsorbate solution and one in 1 mM hexane adsorbate solution are consistent with having a ×2 and 4×2 lattice, respectively. Received: 22 September 1999 / Accepted: 28 March 2000 / Published online: 11 May 2000     

Coherent scattering phenomena in apertureless scanning near-field fluorescence microscopy

We study near-field fluorescence images of samples composed of aggregates of 100 nm dye-doped latex spheres. These images have been performed by a reflection Apertureless Scanning Near-field Optical Microscope (A-SNOM). We show that the near-field distribution in fluorescence A-SNOM images arises from coherent scattering phenomena between all spheres. This is a consequence of the coherent nature of the fluorescence emission of each single sphere. The results shown here are significant for all fluorescent samples characterised by a nonnegligible topography.     

Characterization of laser-ablated and chemically reduced silver colloids in aqueous solution by UV/VIS spectroscopy and STM/SEM microscopy

Silver colloids in aqueous solution were studied by different scanning microscopy techniques and UV/VIS spectroscopy. The silver colloids were produced either by chemical reduction or by nanosecond laser ablation from a solid silver foil in water. Variation of laser power and ablation time leads to solutions of metal clusters of different sizes in water. We characterized the electronic absorption of the clusters by UV/VIS spectroscopy. STM (scanning tunneling microscope) imaging of the metal colloids shows atomic resolution of rod- or tenon-like silver clusters up to 10-nm length formed by laser ablation. Our scanning electron microscope measurements, however, show that much larger silver colloids up to 5-μm length are also formed, which are not visible in the STM due to their roughness. We correlate them with the long-wavelength tail of the multimodal UV/VIS spectrum. The silver colloids obtained by chemical reduction are generally larger and their electronic spectra are red-shifted compared to the laser-ablated clusters. Irradiation of the colloid solution with nanosecond laser pulses of appropriate fluence at 532 nm and 355 nm initially reduced the colloid size. Longer irradiation at 355 nm, however, leads to the formation of larger colloids again. There seems to be a critical lower particle size, where silver clusters in aqueous solution become unstable and start to coagulate. Received: 24 June 2002 / Revised version: 25 July 2002 / Published online: 25 October 2002 RID="*" ID="*"This work is part of the thesis of H. M?ltgen RID="**" ID="**"Corresponding author. Fax: +49-211/811-5195, E-mail: kleinermanns@uni-duesseldorf.de     

Internal reflection mode scanning near-field optical microscopy with the tetrahedral tip on metallic samples

An internal reflection mode is introduced for scanning near-field optical microscopy (SNOM) with the tetrahedral tip. A beam of light is coupled into the tip and the light specularly reflected out of the tip is detected as a photosignal for SNOM. An auxiliary STM mode is used to control the distance during the scanning process and to record the topography of the sample simultaneously with the SNOM image. Images were obtained of different metallic samples which show a contrast in the order of 10% of the total reflected photosignal. In images of metallic samples an inverted contrast is consistently obtained compared to images previously obtained of comparable samples in a transmission mode. The contrast shows a pronounced dependence on the polarization of the incident beam with respect to the orientation of the edges of the tip. In the case of gold surfaces, the photosignal as a function of distance between the tip and the surface shows a pronounced peak in the near-field range of 0–20 nm which is tentatively attributed to the excitation of surface plasmons on the gold surface. The pronounced near-field effects and the strong contrast in the near-field images and the resolution well below 50 nm are an indication of a highly efficient coupling of the incident beam to a local excitation of the tip apex which is essential for the function of the tip as a probe for SNOM. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 21 October 1999     

Gas monitoring in the process industry using diode laser spectroscopy

2 , CO, NH₃, HCl and HF are described together with measurements from several installations. The monitors show continuous measurements with fast response and good sensitivity, all of which is difficult to obtain with conventional techniques such as wet chemical analysis. Received: 19 March 1998/Revised version: 2 June 1998     

Patterning of YBa2Cu3O7-δ films using a near-field optical configuration

2 Cu₃O_7-δ films have been patterned by means of an arrangement employed in scanning near-field optical microscopy (SNOM). Standard SNOM probes were modified to achieve high transmission for direct writing by oxygen depletion in N₂ atmosphere. The written lines are about 1.5 μm wide and show a semiconductor-like resistivity behavior (δ≥0.5). The morphology of illuminated regions is about the same as that of the YBCO films. Received: 3 July 1998/Accepted: 6 July 1998     

Near-field optical microscopy using an integrating sphere

A scanning near-field optical microscope using an integrating sphere is demonstrated. The images from a usual near-field optical microscope often include contrasts caused by the sample structure because the reflection and transmission angles of signal waves depend on the size and shape of the sample. The observation angle dependency of signals can be avoided by using an integrating sphere. Background signals resulting from using the sphere can be reduced by using lock-in detection synchronously with modulation of tip–sample distance. The whole detection system is possibly useful for observing the distribution of the refractive index and/or the absorption coefficient. Received: 27 October 2000 / Final version: 29 August 2001 / Published online: 7 November 2001     

Spectroscopy in our age

The development of modern spectroscopy is summarized from Bunsen’s detection of atoms as the beginning of spectral analysis to modern molecular spectroscopies including new high resolution techniques for molecular ions. Recent experiments involving long range charge migration in peptides and proteins are outlined. Received: 2 October 2000 / Published online: 11 October 2000     

Scanning tunneling microscopy of carbon nanotubes

This article reports on the application of scanning tunneling microscopy for the study of surface structures and electronic properties of carbon nanotubes. Geometric effects resulting from the cylindrical shape of the tubes as well as the particular band structure of the graphitic crystal lattice can lead to a variety of contrast patterns. On the atomic scale, it is sometimes possible to see the full honeycomb lattice structure but often different structures are observed. Besides distortions caused by tip–sample interactions, we find that a complex superstructure superimposed on the simple atomic contrast pattern arises from elastic scattering of the Fermi states at defects or impurities. From a careful analysis of high-resolution images it is possible to extract information about elastic strain of individual tubes. A new combination of scanning tunneling and scanning force microscopy enables near-atomic point resolution of the force signal the tubes can be identified without the need of a conducting substrate. This imaging mode is a crucial step for the characterization of electronic devices based on individual single-wall tubes. This mode can be further enhanced by the use of single-walled tubes as probe tips. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 4 August 1999     

Nanophotonics: a new multidisciplinary frontier

Nanophotonics, defined as nanoscale optical science and technology, is a new multidisciplinary frontier. This article presents our studies on nanoscale matter–radiation interactions utilizing nanoscale confinement of radiation as well as on nanoscale photochemical transformations, particularly nanoscale nonlinear optical interactions. The selected examples of our studies include nanoscopic optical harmonic generation, multiphoton fluorescence, transient absorption dynamics, surface-plasmon-enhanced two-photon fluorescence, and nonlinear optical information storage. Received: 12 December 2001 / Published online: 11 June 2002     

Determining the spin polarization of surfaces by spin-polarized scanning tunneling spectroscopy

During the last few years, spin-polarized scanning tunneling microscopy and spectroscopy has been developed as a reliable tool to image surface magnetic domain structures of bulk materials as well as thin films and nanostructured systems. In principle, this technique also allows for the determination of the energy-resolved spin polarization of the sample P_S(E) with nanometer resolution, information which might play a crucial role in understanding systems like, for example, non-magnetic adatoms on magnetic surfaces. A main problem in quantifying P_S(E), however, arises from the fact that, in contrast to planar junctions, the tip–sample distance generally varies with the magnitude and direction of the surface magnetization, since the distance is controlled indirectly by the tunneling current that is itself spin-polarized. We employ a simple model of the tunneling process to investigate this issue and show that a normalization of the dI/dU spectra with the total conductance I/U is insufficient to correct for their distance dependence. Received: 2 September 2002 / Accepted: 2 September 2002 / Published online: 5 March 2003 RID="*" ID="*"Corresponding author. Fax: +49-40/42838-5311, E-mail: kubetzka@physnet.uni-hamburg.de     

Frequency-modulation-enhanced remote sensing

6 to 10¹² molecules cm^-3. FMRS has the potential of being a general method for monitoring atmospheric trace gases at ambient levels. Received: 23 March 1998     

Prerequisites for high-quality magnetic tunnel junctions: XPS and NMR study of Co/Al bilayers

Aluminum films with thicknesses ranging from 1 nm to 12 nm have been sputtered on 20 nm thick Co layers. The properties of the Co/Al bilayers were studied by X-ray photoemission spectroscopy (XPS) and spin-echo nuclear magnetic resonance (NMR). Both methods show independently that a 1 nm Al film covers the Co surface completely. XPS and NMR also showed that layers thicker than 1 nm Al are not oxidized completely in ambient air. Similarities to and deviations from niobium with Al overlayers (Nb/Al) are described. Prerequisites for the fabrication of tunneling magnetoresistance devices based on Co or NiFe ferromagnets and an aluminum oxide barrier are discussed. Received: 7 July 1999 / Accepted: 11 November 1999 / Published online: 8 March 2000     

Magnetostatic interaction studied by force microscopy in ultrahigh vacuum

3 (FeGe)₅ O₁₂ magnetic garnet, Co/Pt multilayer sample and the high coercivity BaFe₁₂O₁₉ single crystal. We have prepared in UHV by electron beam evaporation magnetic sensors/tips which allowed us to reach high sensitivity and high resolution of our microscope. Ideal UHV conditions (pressure 5×10^-11 \text{Torr}) helped us to observe domain wall contrast on all of studied samples by using a dynamic mode of operation of the force microscope. Received: 28 October 1996/Accepted: 5 November 1996     

Evaluation of structural and adhesive properties of nylon 6 and PTFE alignment films by means of atomic force microscopy

Atomic force microscopy (AFM) has become a powerful technique for submicron investigation of surface properties. In this work we use the capability of this technique to investigate dielectric films used to align ferroelectric liquid crystals (FLC). In fact, the final performance of a surface stabilized FLC (SSFLC) flat panel display strongly depends on the alignment layer properties and quality. This work focuses on a comparison of two alignment films: the more conventional polyamide, nylon 6, and polytetrafluoroethilene (PTFE, commercially known as Teflon), only recently used as a new aligning material. A micromorphological characterization of the sample surfaces has been carried out in order to correlate structure with alignment properties of both polymer films. The results show varying roughness and periodicity wavelengths for the two alignment layers. These different properties can be related to different anchoring forces between aligning surfaces and FLC molecules and therefore to a different electrooptical response of SSFLC cells. In addition to the topographic characterization, AFM non-conventional measurements have been performed on alignment layers deposited on different transparent conductive oxides, such as indium tin oxide (ITO) and SnO₂, used to make electrodes in SSFLC displays. These measurements provide local information on the adhesive properties of the studied alignment materials as a function of substrate coating. These observations indicate less adhesion of PTFE with respect to nylon 6. Received: 16 April 2000 / Accepted: 16 April 2000 / Published online: 13 September 2000     

Application of scanning force microscopy in nanotube science

Recent developments in the application of scanning force microscopy in nanotube science are reviewed. The non-destructive character of this technique allows the structural characterisation of (chemically modified) single- and multi-wall nanotubes deposited on substrates for further investigations such as electrical transport measurements. Furthermore, SFM is now an established tool for manipulation of nanotubes, which allows position control and determination of elastic constants such as the Young’s modulus. Finally it is shown that very sharp and stable probes for scanning force microscopy can be made from nanotubes due to their excellent stability and aspect ratio. Received: 17 May 1999 / Accepted: 18 May 1999 / Published online: 29 July 1999