摘要:

The electromagnetic fields that can build up around metallic or dielectric pointed tips are of increasing interest in context with the new scanning probe microscopies (tunneling, near‐field optics, Coulomb and van der Waals forces etc.). The paper presents exact solutions of Laplace’s equations for the tip/sample geometry. For suitable media, plasmons are found whose electric fields are highly localized in the gap region. We believe that the field enhancement associated with suchtipplasmonsis instrumental for inelastic tunneling and light emission during scanning tunneling microscopy.

ISSN:0734-211X    

DOI:10.1116/1.585558

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The speed limitations conventionally encountered in scanning tunneling microscopy, scanning capacitance microscopy, and atomic force microscopy result from the external electronics and are not inherent to the techniques themselves. Ultrafast time resolution can be achieved through the use of correlation methods. We demonstrate the application of time‐resolved optical correlation techniques to scanned probe microscopy by probing the relaxation of photo‐excited carriers at the Si(111)–(7×7) surface on the nanosecond and picosecond time scales using scanning tunneling and scanning capacitance microscopy measurements of the surface photovoltage. The observed temporal response demonstrates that the voltage arises from photovoltaic effects and does not arise from direct optical rectification in the tunnel junction gap.

ISSN:0734-211X    

DOI:10.1116/1.585559

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We show that the scanning tunneling microscope (STM), operated in ultrahigh vacuum, can be made sensitive to the electron spin and therefore to magnetic structures down to the atomic scale. The experiments were performed with a ferromagnetic CrO2tip providing electrons of high spin polarization. As sample, we used a Cr(001) single crystal which offers an ideal magnetic test structure. The experimental results obtained with the ferromagnetic CrO2tip characteristically differ from those obtained with a nonmagnetic tungsten tip. We give an expression, relating the local electron spin polarization of the tunneling junction with quantities directly measurable with the STM. We also discuss possible contributions from magnetic dipole and exchange forces as well as the feasibility of magnetic exchange force microscopy.

ISSN:0734-211X    

DOI:10.1116/1.585560

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

The tunneling of photons from an evanescent wave to a sharpened optical fiber probe tip provides the basis for an analogous instrument to the electron scanning tunneling microscope. The transmission and reflection coefficients fors‐polarized photons are exactly the same as those for electrons. Imaging at subwavelength resolution is made possible by the exponential character of the tunneling. We have constructed several photon scanning tunneling microscopes (PSTMs) and successfully imaged the topography of a number of samples placed on microscope slides. Each slide is coupled to a prism with an index matching gel and light is internally reflected within the sample. The reflection is frustrated as tunneling sets in when the probe tip is brought close to the surface. The electronics and software used are similar to those used in our electron STMs. Lateral resolution of λ/12 has been obtained and high‐resolution spectroscopy has also been accomplished simultaneously. Images of several types of samples, including biological samples will be presented.

ISSN:0734-211X    

DOI:10.1116/1.585561

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

A report of work in progress to determine the feasibility of imaging the magnetization of ferromagnetic samples with the scanning tunneling microscope (STM) is presented. A vacuum system was designed to test several different proposals as well as to prepare and characterize thin films of magnetic materials by conventional means, including STM. This was begun with an attempt to detect spatially resolved spin‐polarization of secondary electrons emitted from the sample when operating the STM in the scanning field emission mode. Results are currently inconclusive, but encouraging. A weak spin‐polarization signal has been observed corresponding to one of the in‐plane components of magnetization. However, topographic feedthrough has not been fully eliminated as a possible spurious source of contrast.

ISSN:0734-211X    

DOI:10.1116/1.585562

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

A new microscope, the scanning chemical potential microscope (SCPM), has demonstrated the measurement of thermoelectric potential variations with atomic resolution. These measurements can be attributed to atomic scale variations in the surface chemical potential gradient (∂μ/∂T). This capability was experimentally demonstrated by measuring the thermoelectric voltage produced at the tunnel junction between a tip and a heated sample with a modified scanning tunnelling microscope (STM). Registered images of STM current and thermoelectric voltage were acquired by temporally multiplexing current and voltage data from line to line, providing a direct spatial comparison of current and voltage contrast. The results on graphite and molybdenum disulfide demonstrate that the tunnelling and thermoelectric contrast is spatially different on an atomic scale. The experiments are described, and the results of some first order classical calculations are presented.

ISSN:0734-211X    

DOI:10.1116/1.585563

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Infrared laser radiation is coupled via the tip into the tunneling junction of a scanning tunneling microscope. Rectification and difference frequency generation of the laser light caused by the nonlinearity in the current‐voltage characteristic of the junction are observed. Images with atomic resolution are obtained either by recording the rectified signal or by measuring the intensity at the difference frequency when the tip is scanned across the surface. These images are compared with others obtained in the conventional way by observing the tunneling current.

ISSN:0734-211X    

DOI:10.1116/1.585564

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Surface bias voltages induced on a scanning tunnel microscope (STM) junction illuminated with laser radiation are spatially resolved for silicon surfaces. Surface photovoltages of ∼0.3 V for Si(111)‐(7×7) and<0.1 V for Si(001)‐(2×1) are observed with large reductions in the vicinity of surface/subsurface defects associated with midgap states. These reductions, attributed to a variation in the recombination rate, have typical surface screening lengths that are less than those found in the bulk. A small, atomically varying signal of 3–5 mV is also observed and is due to spatial variations in rectification efficiency rather than photovoltage.

ISSN:0734-211X    

DOI:10.1116/1.585565

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

Tunneling spectroscopy has been used to detect the photoexcitation of charge carriers in the wide band‐gap semiconductors, ZnO and cubic SiC. Because the process is energy sensitive, valence‐to‐conduction band or defect charge transfer transitions may be selectively excited and detected with the scanning tunneling microscope. Two types of transitions were detected which change the tunneling response; for cubic SiC valence‐to‐conduction band transitions were excited, while for Co2+and Mn2+doped ZnO electron charge transfer transitions from the dopants to the conduction bands occur. Preliminary results on the effect of a continuous energy (ultraviolet) light source on the tunneling spectrum of ZnO are presented.

ISSN:0734-211X    

DOI:10.1116/1.585566

出版商:American Vacuum Society    

年代:1991

数据来源: AIP

摘要:

We have investigated the light emission mechanisms from a scanning tunneling microscope in which the sample surface is a thin film of Au (22.5 nm thick) evaporated on a microscope cover glass. This sample was attached to the flat surface of a hemicylindrical coupler prism. The tunneling electrons from a Pt‐Ir tip were directed to the Au film from the air side, and the angle dependence of light emission intensity was measured through the coupler prism using a photon counting system. We found a sharp emission peak centered at 43° from the film surface normal. This is the angle at which the fast mode of surface plasmon polaritons (SPP) mostly localized at the Au/air interface can radiate by conserving the wave vector parallel to the interface. Thus we conclude that most of the light on the prism side is emitted by the delocalized fast mode of SPP of the Au/air interface.

ISSN:0734-211X    

DOI:10.1116/1.585567

出版商:American Vacuum Society    

年代:1991

数据来源: AIP