Thin Films Of Potassium Research Articles

Simple metal under tensile stress: layer-dependent herringbone reconstruction of thin potassium films on graphite.

While understanding the properties of materials under stress is fundamentally important, designing experiments to probe the effects of large tensile stress is difficult. Here tensile stress is created in thin films of potassium (up to 4 atomic layers) by epitaxial growth on a rigid support, graphite. We find that this “simple” metal shows a long-range, periodic “herringbone” reconstruction, observed in 2- and 3- (but not 1- and 4-) layer films by low-temperature scanning tunneling microscopy (STM). Such a pattern has never been observed in a simple metal. Density functional theory (DFT)simulations indicate that the reconstruction consists of self-aligned stripes of enhanced atom density formed to relieve the tensile strain. At the same time marked layer-dependent charging effects lead to substantial variation in the apparent STM layer heights.

Non-adiabatic processes in the charge transfer reaction of O2 molecules with potassium surfaces without dissociation

Thin potassium films grown on Si(001) substrates are used to measure internal chemicurrents and the external emission of exoelectrons simultaneously during adsorption of molecular oxygen on K surfaces at 120 K. The experiments clarify the dynamics of electronic excitations at a simple metal with a narrow valence band. X-ray photoemission reveals that for exposures below 5 L almost exclusively peroxide K2O2 is formed, i.e., no dissociation of the molecule occurs during interaction. Still a significant chemicurrent and a delayed exoelectron emission are detected due to a rapid injection of unoccupied molecular levels below the Fermi level. Since the valence band width of potassium is approximately equal to the potassium work function (2.4 eV) the underlying mechanism of exoemission is an Auger relaxation whereas chemicurrents are detected after resonant charge transfer from the metal valence band into the injected level. The change of the chemicurrent and exoemission efficiencies with oxygen coverage can be deduced from the kinetics of the reaction and the recorded internal and external emission currents traces. It is shown that the non-adiabaticity of the reaction increases with coverage due to a reduction of the electronic density of states at the surface while the work function does not vary significantly. Therefore, the peroxide formation is one of the first reaction systems which exhibits varying non-adiabaticity and efficiencies during the reaction. Non-adiabatic calculations based on model Hamiltonians and density functional theory support the picture of chemicurrent generation and explain the rapid injection of hot hole states by an intramolecular motion, i.e., the expansion of the oxygen molecule on the timescale of a quarter of a vibrational period.

Single domain transport measurements ofC60films

Thin films of potassium doped ${\mathrm{C}}_{60},$ an organic semiconductor, have been grown on silicon. The films were grown in ultrahigh vacuum by thermal evaporation of ${\mathrm{C}}_{60}$ onto oxide-terminated silicon as well as reconstructed Si(111). The substrate termination had a drastic influence on the ${\mathrm{C}}_{60}$ growth mode which is directly reflected in the electrical properties of the films. Measured on the single domain length scale, these films revealed resistivities comparable to bulk single crystals. In situ electrical transport properties were correlated to the morphology of the film determined by scanning tunneling microscopy. The observed excess conductivity above the superconducting transition can be attributed to two-dimensional fluctuations.

A study on the influence of the cations and anions on the electrical conductivity in thin films of potassium and rubidium halides

Thin films of rubidium chloride and rubidium iodide have been studied with respect to their transient and AC electrical behaviour at room temperature. The transient current was measured in the time range 10-5 s to 104 s, following the application of a step voltage for different amplitudes between 0.4 V and 2.0 V. The AC current was measured over the frequency range 1 Hz to 200 kHz for harmonically varying voltages with amplitudes in the range 0.4-1.2 V. Analysis of the results shows that the AC conductivity is due to anions, whereas the transient conductivity is due only to anions in the time interval from the onset of the applied step voltage up to 10-2 s. For times greater than 10-2 s the transient conductivity is due not only to anions, but also to the release of trapped electrons by the Poole-Frenkel mechanism alone or assisted by tunnelling. The results are compared with previous studies on potassium halides and rubidium bromide. Finally, the observed behaviour is correlated with the electronic polarizabilities of the ions and the effective charge of the salts.

Size effects in the resistivity of thin films of potassium

The authors have measured size effects in the electrical resistivity of thin films of potassium below 1 K. These size-effect data appear to be highly anomalous when compared with their previous resistivity data for thin wires of potassium. However, they show that both sets of measurements can be explained within a unified theoretical framework in terms of normal electron-electron scattering and normal electron-phonon scattering. Specific predictions of size effects are made for size ranges not covered by the present experimental data.

Zur Bestimmung der Frequenz von Plasmaschwingungen durch optische Messungen an dünnen Metallschichten: Transmissionsmessungen an Kaliumschichten bei den Temperaturen des flüssigen N2 und He

Optical transmission spectra in the region of the volumeplasma-frequencyωp were measured for ⊥ polarized radiations incident on thin potassium films either from the vacuum (front) side at an angle of 45° or from the quartz-support (back) side at about 32°±2° according to wavelength. The films were prepared by evaporation in static ultra-high vacuum (∼10−10 Torr) on a smooth surface of a thin, plane-parallel platelet of polycrystalline quartz cooled either to 10 K or to 80 K, their thickness being exactly measured by means of the Langmuir-Taylor ionisation method. The minimum of the transmission curves givingωp was found independent of thickness for films condensed at 10 K, but decreasing in the same thickness region with decreasing thickness for films condensed at 80 K. This variation as well as the differences between the values of ωp determined by various authors are attributed to varying degrees of surface roughness of the films used.

A calculation of the electrical conductivity of thin films of potassium and sodium

Calculations have been carried out on the electrical conductivity of both perfect and imperfect films of potassium at temperatures between 200K and 20K and of perfect films of sodium at temperatures between 200K and 50K, for two values of the specularity parameter p=0 and p=0.5. In all cases, the Boltzmann equation was solved numerically with a realistic electron-phonon interaction being used in the scattering term. It was found that, for all the above films, for a given value of p, the ratio of the mean resistivity of the film to that of the bulk crystal depends almost exclusively on a single parameter, kappa , the ratio of the film thickness to the effective electron mean free path. While the results of the Fuchs-Sondheimer theory agree with theirs for large values of kappa , the authors find that for smaller values of kappa the conductivity of the films is appreciably less than predicted by this theory. An explanation of this breakdown of the Fuchs-Sondheimer theory is proposed.

The electrical conductivity of thin metal films at low temperatures

The results are presented of calculations of the electrical conductivity of thin films of potassium as a function of the film's thickness and temperature, at temperatures such that the resistance in the body of the film is due mainly to the electron-phonon interaction and the scattering by this mechanism is anisotropic. For these calculations, the Boltzmann equation for the film, with appropriate scattering of the electrons at its surfaces, is converted into a form which can readily be solved by numerical methods. The qualitative theory of Blatt and Satz (1960) is not used, and both normal and umklapp processes are included in the electron-phonon interaction, without recourse to Matthiesen's rule.

Optically Excited Longitudinal Plasmons in Potassium

Oscillatory structure in the absorption of thin potassium films has been observed for energies just above the plasmon energy in a wavelength-modulated photoemission experiment. This structure is shown to be due to resonant excitation of longitudinal plasmons. Observation of up to ten resonances allows the evaluation of the dispersion relation, which shows a quadratic behavior in agreement with a theoretical formula based on the Boltzmann equation.

Plasmaresonanzemission und Plasmaresonanzabsorption an dünnen Kaliumschichten

Optical transmission and emission spectra in the region of the plasma frequencyωp were measured on thin potassium films evaporated on sapphire substrates of various temperatures. For the foils condensed at a temperatures of the substrateTa −165 °C transmission spectra were observed which transmission minima are displaced to frequencies less thanωp. Plasma resonance emission was observed from these foils. Comparison with calculated emission spectra gives an approximate agreement.

Untersuchungen an d�nnen Alkalimetallschichten V

At 10−8 mm Hg thin potassium films are evaporated on a quartz target by means of a potassium atomic beam in a reproducible manner. The big change of the optical absorption and of the electrical conductivity of such films on targets which are either “unbekernt” or “bekernt” is measured as a function of target temperature and film thickness.

Electrical resistivity of thin films of potassium at 100°K

The electrical resistivity of thin potassium films at 100°K has been measured as a function of thickness in the range 148–1600 Å. The experimental results are in excellent agreement with Fuchs's theoretical predictions in the range 500–1600 Å and show that the scattering of the electrons at the film surfaces is completely diffuse. Comparison of experimental and theoretical curves gives a bulk resistivity parameter of 1·85 μΩ-cm, a mean free path of 1180 Å, and an electron to atomic density ratio of 1·0. The films were deposited on a Pyrex substrate from a beam of potassium vapor in a vacuum chamber which had a pressure less than 5 × 10 −8 mm Hg. The film thickness, in each case, was computed from a known beam intensity, exposure time, and film density. A surface ionization detector was used to calibrate the beam intensity ( J 1) as a function of effusion oven temperature ( T 0). The linear plot of log ( J 1 T 0 1 2 ) versus T 0 −1 gave a heat of sublimation for potassium at 450°K of 21·1 kcal mole , in good agreement with Brewer's value of 21·4 kcal mole within our experimental error.

Normal Energy Distribution of Photoelectrons from Thin Potassium Films as a Function of Temperature

Normal Energy Distribution of Photoelectrons from Thin Potassium Films as a Function of Temperature

Calculated and Experimental Photoelectric Emission from Thin Films of Potassium

Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text Herbert E. Ives and H. B. Briggs, "Calculated and Experimental Photoelectric Emission from Thin Films of Potassium," J. Opt. Soc. Am. 26, 247-250 (1936) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article

Energy Distribution of Photoelectrons as a Function of the Thickness of a Potassium Film

Current-voltage curves were taken for photoelectrons ejected from thin potassium films of various thicknesses. The films were formed on a silvered glass surface at room temperature by means of a molecular beam. The condensing surface was a small sphere placed at the center of a large collecting sphere. The thickness of the potassium film was computed from the geometry of the photocell, the temperature and time of heating of the potassium reservoir, and data on the vapor pressure of potassium. The small inner sphere was connected to an electrometer which measured the photocurrent. The contact potential between the film and the silvered surface of the large sphere was determined from the values of the thresholds of the two surfaces. The form of the curves near the maximum retarding potential was approximately the same for each film thickness. The curves were analyzed by DuBridge's method and were in good agreement with the theory. For films less than 3 molecular layers corrections had to be made for a constant decrease in photocurrent after the formation of the film. However, for films of greater thickness the photocurrent was quite constant. The photocurrent for films less than 3 molecular layers failed to saturate for accelerating potentials up to 20 volts. As the film thickness increased the photocurrent began to saturate at lower voltages.

The Distribution in Direction of Photoelectrons from Alkali Metal Surfaces

An experimental study of the distribution in direction of photoelectrons emitted from alkali metal surfaces irradiated by light incident at varions angles and polarized in different planes.---The alkali metal surfaces used were of two sorts: (1) liquid alloys of sodium and potassium, (2) thin films of potassium or rubidium on polished platinum. In all cases the alkali metal surface was at the center of a large spherical enclosing anode, provided either with collecting tabs at various angular positions or with an exploring finger. It is found that the emission closely obeys Lambert's law, but that the ellipse by which the emission is represented, in polar coordinates, is more elongated normally to the surface for perpendicularly incident light than for obliquely, when the direction of the electric vector is in both cases parallel to the surface, and still more elongated for obliquely incident light with the electric vector in the plane of incidence. The distribution curves are all perfectly symmetrical about the normal to the surface, showing no tendency to follow the direction of the electric vector.