Ultrasharp tungsten tips—characterization and nondestructive cleaning

Setvín, Martin; Javorský, Jakub; Turčinková, Dana; Matolı́nová, Iva; Sobotík, P.; Kocán, Pavel; Ošt’ádal, Ivan

doi:10.1016/j.ultramic.2011.10.005

Cited by 34 publications

(22 citation statements)

References 23 publications

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“…The AFM/STM measurements were performed in an UHV chamber with a base pressure below 2 × 10 −11 mbar, equipped with a ScientaOmicron LTSTM/AFM at T=4.8 K, using the q-Plus setup with a separate wire [33] for the tunneling current and a differential cryogenic preamplifier [34]. Etched tungsten tips were glued on the tuning fork and cleaned by self-sputtering in Ar atmosphere [35] prior to the experiment. The resonance frequency of the used qPlus cantilevers was 77 kHz with the Q factor of ≈ 50000.…”

Section: Methodsmentioning

confidence: 99%

Incipient ferroelectricity: A route towards bulk-terminated SrTiO3

Sokolović

Schmid

Diebold

et al. 2019

Phys. Rev. Materials

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Perovskite oxides attract increasing attention due to their broad potential in many applications [1][2][3]. Understanding their surfaces is challenging, though, because the ternary composition of the bulk allows for multiple stable surface terminations [4]. We demonstrate a simple procedure for preparing the bulk-terminated (001) surface of SrTiO 3 , a prototypical cubic perovskite. Controlled application of strain on a SrTiO 3 single crystal results in a flat cleavage with micrometer-size domains of SrO and TiO 2 . Distribution of these two terminations is dictated by ferroelectric domains induced by strain [5] during the cleavage process. Atomically-resolved scanning tunneling microscopy/atomic force microscopy (STM/AFM) measurements reveal the presence of point defects in a well-defined concentration of (14±2)%; Sr vacancies form at the SrO termination and complementary Sr adatoms appear at the TiO 2 termination. These intrinsic defects are induced by the interplay between ferroelectricity, surface polarity, and surface charge.SrTiO 3 is the prototypical cubic perovskite oxide, interesting for its catalytic and photocatalytic properties [6], potential use in oxide electronics [7][8][9][10], and fundamental questions including the appearance of twodimensional electron gas at its surfaces [11][12][13] and interfaces [3]. The SrTiO 3 (001) surface plays a key role in all these functionalities, but the available surface studies of this facet illustrate a general problem of approaching perovskite materials: A plethora of various surface terminations can form, depending on the preparation technique and on slight changes in the surface stoichiometry. The centre of interest typically lies in bulk-terminated perovskite surfaces, because wet chemical preparation methods typically provide surfaces with a (1×1) diffraction pattern [9]. Yet this issue is contoversial due to the possible presence of amorphous structures [14] or contaminants [15].Cleaving a single crystal would be an obvious solution for obtaining a pristine surface. Despite numerous attempts [16-18], we are not aware of any successful work leading to atomically flat bulk-terminated SrTiO 3 . The main impediment is that cubic perovskites typically do not possess a natural cleaving plane. Instead they exhibit so called conchoidal fracture (see Figure S1 in Supplementary Information). An alternative technique for surface preparation is applying cycles of sputtering and high-temperature annealing -a standard procedure used for preparing oxide surfaces in ultrahigh vacuum (UHV). For perovskite surfaces, however, this methods results into a series of complex reconstructions [4,19,20], which are very stable and chemically inert; exactly the opposite to the behaviour of perovskites in most applications.Our previous study on KTaO 3 [21] indicated that ferroelectricity induced in a perovskite can create a natural cleavage plane oriented perpendicular to the electric polarization. SrTiO 3 is an incipient ferroelectric material: Its ferroelectric Curie temperature lies...

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Section: Methodsmentioning

confidence: 99%

Incipient ferroelectricity: A route towards bulk-terminated SrTiO3

Sokolović

Schmid

Diebold

et al. 2019

Phys. Rev. Materials

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“…O 2 was dosed directly into the cryostat at T = 5.5 K. Tuning-fork-based AFM sensors with a separate wire for the tunneling current were used (51) (k = 3,750 N/m, f 0 = 47,500 Hz, Q ≈ 50,000). Electrochemically etched W tips were glued to the tuning fork and cleaned in situ by field emission and self-sputtering in 10 −4 Pa Ar (52). The reactive tip termination was reproducibly obtained by touching the anatase surface and applying a high tunneling current (53).…”

Section: Methodsmentioning

confidence: 99%

Electron transfer between anatase TiO ₂ and an O ₂ molecule directly observed by atomic force microscopy

Setvín

Hulva

Parkinson

et al. 2017

Proc. Natl. Acad. Sci. U.S.A.

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Activation of molecular oxygen is a key step in converting fuels into energy, but there is precious little experimental insight into how the process proceeds at the atomic scale. Here, we show that a combined atomic force microscopy/scanning tunneling microscopy (AFM/STM) experiment can both distinguish neutral O molecules in the triplet state from negatively charged (O) radicals and charge and discharge the molecules at will. By measuring the chemical forces above the different species adsorbed on an anatase TiO surface, we show that the tip-generated (O) radicals are identical to those created when () an O molecule accepts an electron from a near-surface dopant or () when a photo-generated electron is transferred following irradiation of the anatase sample with UV light. Kelvin probe spectroscopy measurements indicate that electron transfer between the TiO and the adsorbed molecules is governed by competition between electron affinity of the physisorbed (triplet) O and band bending induced by the (O) radicals. Temperature-programmed desorption and X-ray photoelectron spectroscopy data provide information about thermal stability of the species, and confirm the chemical identification inferred from AFM/STM.

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“…Finally, nc-AFM data were taken in a two-vessel UHV setup (preparation chamber <10 –10 mbar, analysis chamber <10 –11 mbar) based on a commercial Omicron LT-STM equipped with a commercial Omicron q-Plus LT head and tuning-fork-based AFM sensors ( k = 1900 N/m, f 0 = 30500 Hz, Q ≈ 20 000). Electrochemically etched W tips were glued to the tuning fork and cleaned in situ by self-sputtering in 10 –4 Pa Ar, 35 followed by treatment on a Cu(100) crystal. While imaging the hematite surface, the tip termination was most likely modified by touching the oxide surface.…”

Section: Experimental and Computational Detailsmentioning

confidence: 99%

Atomic-Scale Structure of the Hematite α-Fe₂O₃(11̅02) “R-Cut” Surface

et al. 2018

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The α-Fe2O3(11̅02) surface (also known as the hematite r-cut or (012) surface) was studied using low-energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), and ab initio density functional theory (DFT)+U calculations. Two surface structures are stable under ultrahigh vacuum (UHV) conditions; a stoichiometric (1 × 1) surface can be prepared by annealing at 450 °C in ≈10–6 mbar O2, and a reduced (2 × 1) reconstruction is formed by UHV annealing at 540 °C. The (1 × 1) surface is close to an ideal bulk termination, and the undercoordinated surface Fe atoms reduce the surface bandgap by ≈0.2 eV with respect to the bulk. The work function is measured to be 5.7 ± 0.2 eV, and the VBM is located 1.5 ± 0.1 eV below EF. The images obtained from the (2 × 1) reconstruction cannot be reconciled with previously proposed models, and a new “alternating trench” structure is proposed based on an ordered removal of lattice oxygen atoms. DFT+U calculations show that this surface is favored in reducing conditions and that 4-fold-coordinated Fe2+ cations at the surface introduce gap states approximately 1 eV below EF. The work function on the (2 × 1) termination is 5.4 ± 0.2 eV.

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Ultrasharp tungsten tips—characterization and nondestructive cleaning

Cited by 34 publications

References 23 publications

Incipient ferroelectricity: A route towards bulk-terminated SrTiO3

Incipient ferroelectricity: A route towards bulk-terminated SrTiO3

Electron transfer between anatase TiO ₂ and an O ₂ molecule directly observed by atomic force microscopy

Atomic-Scale Structure of the Hematite α-Fe₂O₃(11̅02) “R-Cut” Surface

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Ultrasharp tungsten tips—characterization and nondestructive cleaning

Cited by 34 publications

References 23 publications

Incipient ferroelectricity: A route towards bulk-terminated SrTiO3

Incipient ferroelectricity: A route towards bulk-terminated SrTiO3

Electron transfer between anatase TiO 2 and an O 2 molecule directly observed by atomic force microscopy

Atomic-Scale Structure of the Hematite α-Fe2O3(11̅02) “R-Cut” Surface

Contact Info

Product

Resources

About

Electron transfer between anatase TiO ₂ and an O ₂ molecule directly observed by atomic force microscopy

Atomic-Scale Structure of the Hematite α-Fe₂O₃(11̅02) “R-Cut” Surface