We study how the (100) surface of magnetite undergoes oxidation by monitoring its morphology during exposure to oxygen at ~650 °C. Low-energy electron microscopy reveals that magnetite's surface steps advance continuously. This growth of Fe3O4 crystal occurs by the formation of bulk Fe vacancies. Using Raman spectroscopy, we identify the sinks for these vacancies, inclusions of α-Fe2O3 (hematite). Since the surface remains magnetite during oxidation, it continues to dissociate oxygen readily. At steady state, over one-quarter of impinging oxygen molecules undergo dissociative adsorption and eventual incorporation into magnetite. From the independence of growth rate on local step density, we deduce that the first step of oxidation, dissociative oxygen adsorption, occurs uniformly over magnetite's terraces, not preferentially at its surface steps. Since we directly observe new magnetite forming when it incorporates oxygen, we suggest that catalytic redox cycles on magnetite involve growing and etching crystal.

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This article presents a study of the influence of the sputtering parameters on the preferred orientation of polycrystalline aluminum nitride thin films. Aluminum nitride films were grown by rf reactive sputtering of an aluminum target in an N2/Ar gas mixture for different values of the deposition parameters: total pressure, nitrogen content in the discharge gas, and substrate bias voltage. The preferred orientation of the films was analyzed by x-ray diffraction. Films with different preferred orientations were obtained, ranging from c-axis oriented films to films with the c axis tilted by up to 61.6° from the substrate normal. The different mechanisms influencing the preferred orientation of the films have been considered, especially the transfer of energy to the adatoms on the substrate by particle bombardment. An analysis of the relation between the deposition parameters and the crystal orientation has allowed us to determine the relative importance of the different particles in the supply of energy to the substrate. We have found that Ar ion bombardment of the film during growth is the most influential mechanism on the preferred orientation of the films. As bombardment becomes more energetic, microcrystals in the film tend to grow with the c axis along the surface normal. The energy of Ar bombardment can be best controlled through the substrate bias voltage, a characteristic that we have employed to obtain AlN films exhibiting pure (00.2) preferred orientation and rocking curves with a full width at half maximum as low as 4.2°.

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Articles you may be interested inSynthesis and characterization of 10nm thick piezoelectric AlN films with high c-axis orientation for miniaturized nanoelectromechanical devices Appl. Phys. Lett.

Using low-energy electron diffraction, we show that the room-temperature ( √ 2 × √ 2)R45 • reconstruction of Fe 3 O 4 (100) reversibly disorders at ∼450 • C. Short-range order persists above the transition, suggesting that the transition is second order and Ising-like. We interpret the transition in terms of a model in which subsurface Fe 3+ is replaced by Fe 2+ as the temperature is raised. This model reproduces the structure of antiphase boundaries previously observed with scanning tunneling microscopy, as well as the continuous nature of the transition. To account for the observed transition temperature, the energy cost of each charge rearrangement is 82 meV. Metal oxides are often useful because of their stability at high temperatures. An example is magnetite, Fe 3 O 4 . In catalytic applications such as the water-gas shift reaction, 1 magnetite is used at temperatures between 300 and 500 • C. Furthermore, magnetite's high Curie temperature of 580 • C allows spintronic applications. 2 Since such applications frequently depend on surface properties, a natural question arises-how does the surface structure change with temperature?The room-temperature properties of magnetite's surfaces are complex. Its (100) surface has been extensively studied. [3][4][5][6][7][8][9][10][11][12][13][14][15] Instead of the (1 × 1) bulklike termination, it reconstructs into a structure with a larger ( √ 2 × √ 2)R45 • unit cell. The atomic structure of this reconstruction has been painstakingly unraveled by density functional theory (DFT), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM) 4 -the surface is terminated by octahedrally coordinated iron atoms arranged in rows, as shown in Fig. 1(a). The observed periodicity results from small displacements of the iron atoms perpendicular to the rows [see Fig. 1(b)]. The driving force for the reconstruction is believed to be ordering of the charge state of the iron in octahedral sites beneath the surface. 15,16 In bulk magnetite at room temperature the average charge state of octahedral iron is +2.5e. The subsurface charge ordering involves disproportionation of this charge into more positively and negatively charged sites. It has been proposed 15,16 that the disproportionation is greatest in the plane of octahedral iron beneath the top layer. Figure 1(c) sketches the proposed charge order in this subsurface layer. In Fig. 1(c), and in our discussion below, the two charge states in the second layer are labeled by their nominal oxidation state Fe 3+ and Fe 2+ , although the charges are estimated to only differ by 0.2-0.4 e rather than e. 15,17 The top layer octahedral iron is displaced in the surface plane to decrease the distance to the nearest subsurface Fe 2+ , giving the undulating rows of surface octahedral iron observed by LEED and STM.What might happen to such a structure when the temperature is raised? One possibility is that the high-temperature, high-entropy surface differs significantly in stoichiometry and termination from the charge ordered ( √...

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