S. F. Alvarado

A differential interferometer for force microscopy

We present a polarizing optical interferometer especially developed for force microscopy. The deflections of the force‐sensing cantilever are measured by means of the phase shift of two orthogonally polarized light beams, both reflected off the cantilever. This arrangement minimizes perturbations arising from fluctuations of the optical path length. Since the measured quantity is normalized versus the reflected intensity, the system is less sensitive to intensity fluctuations of the light source. The device is especially well suited to static force measurements. The total rms noise measured is ≲0.01 A in a frequency range from 1 Hz to 20 kHz.

Understanding magnetic force microscopy

Magnetic force microscopy is a new method for imaging ferromagnetic domains with a high lateral resolution (10 nm). In this paper we give the basic tip parameters that have to be taken into account to achieve a quantitative image interpretation. For the electrochemically otched polycrystalline iron, nickel and cobalt wires, the tip-apex domain is found to be oriented along the tip axis, because of shape anisotropy. The stray field emerging from the tip apex is comparable to the size of the tip saturation field. The effective domain lengthL determines the image formation: the force due to magnetization patterns of scales which are large compared toL follow the point-dipole approximation. In the opposite case, a single-pole model is more appropriate. While a cobalt tip can be treated as an isolated domain, for nickel and iron a net polarization in the tip wire induced by the front apex-domain has to be considered. A new analytical theory provides an overall understanding of the image formation and allows the determination of the magnetic field vector and the estimation of its magnitude from measurements.

Luminescence in scanning tunneling microscopy on III–V nanostructures

Using electroluminescence associated with scanning tunneling microscope in AlxGa1−xAs heterostructures, we show that: (a) luminescence due to recombination can be induced within single quantum wells of dimensions down to a few nm and can also be used to image them, (b) the energy of bulk bands can be determined, and (c) transport parameters can be measured, e.g., the thermalization length and the diffusion length of minority electrons. This technique opens up new possibilities for the study and characterization of semiconductors and devices—including the possibility of studying surface states and single trapping centers associated with lattice defects, impurities, chemisorbed species, etc.

Separation of magnetic and topographic effects in force microscopy

Several techniques are presented which allow magnetic force microscopy to be performed while simultaneously mapping the surface topographic features of a magnetic sample. The separation of magnetic and topographic features measured simultaneously with a scanning force microscope is made possible by an instrument based on a differential interferometer that can detect cantilever deflections of 0.005 nm at a frequency as low as 1 Hz. Two different applications are presented.

Experimental study of the magnetic critical behavior of the Ni(001) and Ni(110) surfaces (invited)

Dedicated to Professor S. Methfessel, upon the occasion of his sixtieth birthday. The critical behavior of the surface magnetization of atomically clean Ni surfaces has been studied by spin polarized low energy electron diffraction. The critical exponent β1 of the order parameter—surface magnetization—is determined from the temperature dependence of the magnetic exchange scattering asymmetry for the specularly reflected beam at various angles of incidence and kinetic energies of the primary beam. For the (001) surface we find β1=0.8 and for the (110) surface β1=0.79, with a statistical uncertainty of ±0.02. The difference in the critical exponent for the two surfaces is thus within experimental accuracy. This indicates that the larger surface relaxation of the (110) surface does not significantly influence the value of the critical exponent β1 in the temperature range 0.002≲1−T/TC≲0.1. The experimental values of the critical exponent β1 are slightly smaller than the predictions of e‐expansion calculatio...

Simultaneous probing of exchange and spin-orbit interaction in spin polarized low energy electron diffraction from magnetic surfaces

An experimental set-up allowing simultaneous probing of exchange and spin-orbit scattering in spin polarized electron diffraction (SPLEED) from magnetic surfaces is described. In a theoretical analysis it is made clear why the measured scattering asymmetries are, in very good approximation, linear superpositions of the exchange and spin-orbit asymmetries. As an illustration SPLEED data for Ni (001) collected at various energies and angles of incidence of the primary beam, are presented and shown to be in good agreement with theoretical results.

STM-excited luminescence on organic materials

Abstract A scanning tunneling microscope (STM) is used to generate electroluminescence from thin films of tris(8-hydroxyquinolato) aluminum (Alq 3 ) deposited on Au(111) substrates. The emission spectra are highly dependent on the local structural features of the thin film. Furthermore, the intensity distribution is modulated by the collective charge-carrier excitations of the tip and the substrate. This is manifested as a red-shifted emission spectrum characterized by relatively narrow emission lines from the organic material. The luminescence signal appears to be a linear superposition of two different kinds of Alq 3 . We show also that the energy of the lowest unoccupied molecular orbital (LUMO) can be determined with the aid of measurements of the threshold for tunneling-excited electroluminescence. For Alq 3 we find E LUMO = 1.57 ± 0.15 eV above the Fermi level.

Scanning tunneling microscopy as a tool to study surface roughness of sputtered thin films

A three‐dimensional image of the surface roughness of four conducting iron‐oxide Fe3O4 thin films was obtained using a scanning tunneling microscope. We obtain grain size and surface roughness of films deposited on Si(100) by reactive sputtering at different substrate temperatures. The apparent grain size lies between 10 and 50 nm, and depends on the substrate temperature and film thickness. We have also determined the scanning tunneling microscopy parameters (tip size and shape) to obtain ‘‘real’’ images (i.e., images without artifacts) of the films.

Influence of alkoxy substituents on the exciton binding energy of conjugated polymers

Abstract Scanning tunneling microscope (STM) spectroscopy and optical absorption measurements were used to determine the exciton binding energy (Eb) of poly(p-phenylenevinylene) (PPV). We find Eb=0.48±0.14 eV, which is significantly higher than the value of Eb reported previously for an alkoxy-PPV derivative. Furthermore, an analysis of photoluminescence (PL) and STM excited cathodoluminescence (CL) spectra, performed on PPV and on alkoxy-PPV derivatives, reveals that the energy separation between the dominant peaks in the vibronic progression is significantly smaller in the alkoxy-substituted compounds. The reduction of Eb appears to be related to the influence of the alkoxy side-chains, inducing a reduction of both the Coulomb interaction as well as the molecular structural relaxation energy of the exciton.

Covalency in the electronic structure of Fe3O4: An ultraviolet inverse photoemission investigation

The unoccupied electronic structure of an opend-shell transition metal oxide, namely Fe3O4, has been addressed by measuring ultraviolet angle-integrated inverse photoemission (IP) spectra acquired in the isochromat mode (hv=10.2-24 eV). Exploitation of photon energy dependence of symmetry-projected IP cross-sections and comparison with the O 1s X-ray absorption spectrum allow us to recognize a strong covalent admixture of Fe [3d; 4 (sp)]-and O (2p)-derived states in this compound.