K. Dransfeld

Heat transfer between two metallic surfaces at small distances

In scanning thermal microscopy, but also in scanning tunneling microscopy, the thermal contact between tip and sample plays an important role. The heat transfer across the vacuum gap between two parallel metallic surfaces, if the gap width is decreased below several microns, has been investigated. At these distances propagating electromagnetic modes die out but simultaneously a transfer of nonpropagating surface modes across the gap becomes more probable. The heat conductance of the vacuum gap should become distance dependent and larger than that given by the Stefan–Boltzmann law; however, the experimental results and theoretical considerations indicate that the heat transfer, based on the discussed proximity mechanism, is very small, smaller than predicted by the theory of Polder and Van Hove [Phys. Rev. B 4, 3303 (1971)].

Detection of surface plasmons by scanning tunneling microscopy

The influence of surface plasmons excited in a polycrystalline silver film on the tunneling current of a scanning tunneling microscope (STM) has been analyzed. The plasmons cause an additional flow of electrons from the tungsten tip to the silver surface on the order of up to 50 pA. This process is independent of the polarity of the applied bias voltage, thereby excluding effects of thermal expansion. The different nature of the ordinary tunneling current and the surface plasmon induced current is clearly revealed by their different dependence on the gap distance. The local distribution of the intensity of the surface plasmon induced signal reveals structures on a nanometer scale. Some of them are correlated to the surface topography.

Scanning Tunneling Microscopy and Photons

Two different approaches to study the interaction of light with the electronic tunneling in a Scanning Tunneling Microscope (STM) will be discussed. If the surface of the semiconducting WSe2 is irradiated with visible light a surface photovoltage and a corresponding photocurrent can be observed. To investigate this effect the influence of light on the I‐V spectroscopy and the wavelength dependence of the photocurrent have been analyzed. The effect of the surface plasmons excited in a thin silver film on the tunneling process has been studied. The surface plasmons provide a very efficient coupling of the incoming light to the tunneling gap. The detection of the surface plasmons is due to the rectification of the optical ac‐field.

Energy-exchange processes by tunneling electrons

Resistive heating, emission heating or cooling (e.g., the Nottingham effect), and thermal fluctuation radiation are examples of energy exchange processes which are fundamental in electron field emission and in tunneling junctions of scanning tunneling microscopy. These exchange processes are analyzed for both electronic tunneling processes. We first discuss the energy delivered by a monoatomic tip in the field emission process. Strong phonon excitation is expected for field emission currents exceeding 1 nA. Secondly we present a theoretical calculation of the thermal deposition associated with the Nottingham effect in a tunneling junction. The calculation is based on the free electron model for the electrode materials and the tunneling process across a planar vacuum gap. Our results show that the thermal power is deposited not only at the electron receiving electrode but also at the emitting electrode. This originates from a finite probability for electrons below the Fermi level to tunnel through the tunneling barrier replaced by electrons starting from the Fermi level. The comparison between the calculations and the recent STM measurements is given. Finally we discuss the other energy exchange processes in the tunneling junction, and conclude that the thermal coupling between the tip and the sample of STM is extremely small under UHV conditions. This is important for high temperature STM.

Dielectric and piezoelectric properties of very thin films of VDF-TrFE copolymers

Abstract Very thin VDF-TrFE copolymer films were prepared by spin coating ranging in thickness down to 100 nm. Their dielectric and ferroelectric behaviour was measured as a function of thickness. Furthermore their surface roughness and their local piezoelectric properties were analysed by using a scanning tunneling microscope.

Thermal sensors for investigation of heat transfer in scanning probe microscopy

Planar thermocouples designed for investigation of heat transfer in scanning tunneling microscopy and scanning thermal microscopy are described. The limit of sensitivity to local thermal power can be as small as 10 nW. The devices are based on two different thin films formed as a cross on a thin glass substrate. Heat fluxes in the cross point can be detected by measuring the thermoelectric signal from two ends of the cross. As described elsewhere planar thermocouples of this type have been successfully used to detect the energy which is deposited by tunneling electrons and to measure the heat which is coupled across a submicron vacuum gap between two metals by the fluctuating fields of electromagnetic surface modes.

Polarization profiles of polyvinylidene fluoride films polarized by a focused electron beam

The depth profiles of the polarization in films of polyvinylidene fluoride (PVDF) as well as in vinylidene‐fluoride–trifluoroethylene (VDF‐TrFE) copolymer films polarized by a focused electron beam were investigated using the piezoelectrically generated pressure step method. The dominant polarization exhibits a broad maximum inside the film. The position of this maximum depends not only on the energy of the incident electrons but also on the material parameters of the sample. Close to the surface exposed to the electron beam we have in addition observed a small secondary maximum of opposite polarization (amounting to about 1 mC/m2). A qualitative model is presented for the poling of films of PVDF and its copolymers with TrFE by focused electron beam accounting for most of the observed features. The application of electron beams for the poling of ferroelectric films allows the production of piezoelectric bimorphs. By using a well‐focused electron beam also ferroelectric domains of very small lateral dimensions can be created which could become important for ferroelectric data storage.

Fast polarization of a ferroelectric polymer on a microscopic scale

Abstract Using the tip of a scanning tunneling microscope thin films of the copolymer poly(vinylidenefluoride-trifluoroethylene) have been locally polarized. The size of the poled area could be reduced to 2·10−14 m2. Switching times down to 100 ns have been found.

Temperature dependence of the piezoelectric constant of thin PVDF and P(VDF-TrFE) films

Abstract The piezoelectric properties of polyvinylidenfluoride PVDF and vinylidenfluoride-trifluoroethylene copolymer films were measured with the Scanning Nearfield Acoustic Microscope SNAM, a non-touching profilometer. This technique enables us to investigate the topography and the local piezoactivity of the polymer films. The piezoelectric constant was determined by monitoring the surface motion stimulated by a low frequency electric field. The piezoactivity as well as the surface structure of the polymer film show a strong temperature dependence.

Atomic scale variation of current noise on GaAs(110) detected by a scanning tunneling microscope

Abstract The spatial distribution of the fluctuations of the tunneling current occurring in a scanning tunneling microscope has been measured. The experiment has been performed on a cleaved (110) surface of GaAs in ultrahigh vacuum. The observed noise pattern reveals a commensurate periodic structure on an atomic scale. The maxima of the noise are located near the Ga-atoms while the maxima in the topography are located either near the Ga- or the As-atoms, depending on the polarity of the applied bias. We propose that the enhanced current noise is caused by fluctuations of the phonon density for a specific mode of surface phonons. The detection of these fluctuations appears to be enabled by the almost planar coordination of the Ga-atoms at the relaxed surface which leads to an increased vibrational amplitude of the Ga-atoms.