Biased Atomic Force Microscope Tip Research Articles

Polarization retention in ultra-thin barium titanate films on Ge(001)

We investigate polarization retention in 10 to 19 nm thick ferroelectric BaTiO3 (BTO) grown on Ge(001) by molecular beam epitaxy. The out-of-plane direction and reversibility of electric polarization were confirmed using piezoresponse force microscopy. After reverse-poling selected regions of the BTO films to a value P with a biased atomic-force microscope tip, we monitored relaxation of their net polarization for as long as several weeks using optical second-harmonic generation microscopy. All films retained reversed polarization throughout the observation period. 10 nm-thick BTO films relaxed monotonically to a saturation value of 0.9 P after 27 days and 19 nm films to 0.75 P after 24 h. Polarization dynamics are discussed in the context of a 1D polarization relaxation/kinetics model.

Topotactic changes on η-Mo4O11 caused by biased atomic force microscope tip and cw-laser

We present topotactic changes on Mo4O11 crystals induced by a biased atomic force microscope tip and continuous laser. The transformation does not change the topography of the samples, while the surface potential shows remarkable changes on areas where the biased AFM tip was applied. No structural changes were observed by Raman spectroscopy, but AFM scans revealed changes to surface potential due to laser illumination. The observed phenomenon could be potentially useful for memristive memory devices considering the fact that properties of other molybdenum oxides vary from metallic to insulators.

Characterization of surface modification in atomic force microscope-induced nanolithography of oxygen deficient La0.67Ba0.33MnO3−δ thin films

We report our studies of the nanolithographic surface modifications induced by an Atomic Force Microscope (AFM) in epitaxial thin films of oxygen deficient Lanthanum Barium Manganese Oxide (La0.67Ba0.33MnO3−δ). The pattern characteristics depend on the tip voltage, tip polarity, voltage duration, tip force, and humidity. We have used Electron Energy Dispersive X-Ray Spectroscopy (EDS) to analyze the chemical changes associated with the surface modifications produced with a negatively biased AFM tip. A significant increase in the oxygen stoichiometry for the patterned regions relative to the pristine film surface is observed. The results also indicate changes in the cation stoichiometry, specifically a decrease in the Lanthanum and Manganese concentrations and an increase in the Barium concentration in the patterned regions.

Electric control of straight stripe conductive mixed-phase nanostructures in La-doped BiFeO3

This study examines the atomic force microscope (AFM) tip-based electrical formation of tens of microns long stripe (and ∼100 nm wide) inorganic one-dimensional nanostructures based on the morphotropic phase boundary of La-doped BiFeO3 epitaxial thin films. The substitution of Lanthanum into bismuth ferrite not only produces the formation of straight stripe mixed-phase patterns but also improves the spatial continuity drastically by two orders of magnitude. We create, switch and erase stripe nanostructures in a reversible and deterministic way. We demonstrate that electrically formed areas with a nearly single variant alignment can be overwritten with different alignments repeatedly, which reflects the reversible and nonvolatile nature of the switching process. In addition, we explore the functionality of the created nanostructures by clarifying ferroelectric polarizations and observing the improvement of the electronic conduction at the phase boundary. Our findings provide new pathways to one-dimensional rewritable nanostructures and inspire researchers to conceive various multifunctional devices by combining the superb electromechanical property with their unique interfacial electronic conduction properties at nanoscale phase boundaries. Scanning a La-doped BiFeO3 epitaxial thin film with a biased AFM tip, we produce straight stripe one-dimensional nanostructures in which two competing polymorphic ferroelectric phases appear alternately at the interval of ∼100 nm showing the enhancement of electronic conduction at the phase boundaries. We can create, switch and erase the eight-variant stripe nanostructures in a reversible and deterministic way by controlling the tip scanning direction and tip biased voltage. The findings provide a new pathway into one-dimensional nanostructures and versatile patterns of ferroelectric domains and interfaces. Some materials exhibit ‘morphotropic phase boundaries’ at which their crystal structure abruptly changes from a tetrahedral to a rhombohedral lattice. Such phase boundaries are created under high pressure or strain and are accompanied by a change in the material's characteristics. For example, there is great interest in controlling mixed-phase domains composed of ‘T’ and ‘R” phases in thin films of bismuth ferrite (BiFeO3, also known as BFO) for electromechanical applications. Now, a South Korea-Australia-based team led by Chan-Ho Yang has used an atomic force microscope tip to create - and further manipulate or erase - well-aligned, alternating straight stripes of the T and R' domains on a BFO thin film doped with lanthanum. In the absence of lanthanum, the stripes are curved rather than straight. The T-R' phase boundary shows enhanced electronic conduction compared to the bulk phases. These precise and rewritable nanostructures might serve to develop functional materials.

Conductance maps of quantum rings due to a local potential perturbation

We performed a numerical simulation of the dynamics of a Gaussian shaped wavepacket inside a small sized quantum ring, smoothly connected to two leads and exposed to a perturbing potential of a biased atomic force microscope tip. Using the Landauer formalism, we calculated conductance maps of this system in the case of single and two subband transport. We explain the main features in the conductance maps as due to the AFM tip influence on the wavepacket phase and amplitude. In the presence of an external magnetic field, the tip modifies the ϕ0 periodic Aharonov–Bohm oscillation pattern into a ϕ0/2 periodic Al’tshuler–Aronov–Spivak oscillation pattern. Our results in the case of multiband transport suggest tip selectivity to higher subbands, making them more observable in the total conductance map.

High-field chemistry of organometallic precursors for direct-write of germanium and silicon nanostructures

The fabrication of integrated multimaterial structures with nanoscale accuracy is a challenging pursuit essential for novel advances in electronics and photonics. The atomic force microscope precisely controls the synthesis of these nanostructures by triggering chemical reactions at determined substrate locations. Using a biased atomic force microscope tip, we have recently demonstrated the localized synthesis of germanium and silicon from organometallic liquid precursors. This localized synthesis yields the direct-write of germanium and silicon nanostructures onto the substrate. Here, we elucidate the reactions occurring during this synthesis in the tip–sample proximity. Reactions for both precursors are triggered by field-emitted electrons that fragment the precursor molecules into the desired products. In this reaction pathway, the synthesis of germanium is more favourable than the synthesis of silicon from precursors containing the same ligands. We explain these differences by comparing experimental data of reaction onset and kinetics to continuum/VASP simulations. This data is supplemented with electron impact mass spectrometry of the precursors and compared with the simulation results.

Charging of nanostructured and partially reduced graphene oxide sheets

We report on the charging of individual graphene oxide (GO) sheets with varied degrees of reduction by using electrically biased atomic force microscope (AFM) tips. AFM measurements indicate that the apparent height of reduced GO (rGO) sheets increases sharply after charging, while the charging ability is enhanced when the GO sheets are deeply reduced. In addition, the rGO sheets tend to be extracted with electrons (or to be injected with holes) with a positively biased AFM tip, in contrast to that with a negatively biased tip. Charging on isolated areas with tunable shape and size on single-layered GO has also been achieved.

Nanoscale stoichiometric modifications and surface charge patterning of La1.975Sr0.025CuO4+δ crystals with a biased atomic force microscope tip

The application of a positive or negative local bias to the surface of La1.975Sr0.025CuO4+δ (LSCO) crystal by a conducting atomic force microscope tip results in accumulation of a (positive or negative, respectively) metastable charge on the surface. The surface charge initially shows diffusive dynamics with a timescale of hours, but thereafter it is shown to be stable for months. The charged regions are found by Auger electron spectroscopy to have a different stoichiometry from the surrounding material. Apart from fundamental implications for the heterostructure device construction, such surface charge manipulation could lead to AFM nanopatterning of superconducting nanoscale devices and applications in memories.

Spatially resolved analysis of edge-channel equilibration in quantum Hall circuits

We demonstrate an innovative quantum Hall circuit with variable geometry employing the movable electrostatic potential induced by a biased atomic force microscope tip. We exploit this additional degree of freedom to identify the microscopic mechanisms that allow two co-propagating edge channels to equilibrate their charge imbalance. Experimental results are compared with tight-binding simulations based on a realistic model for the disorder potential. This work provides also an experimental realization of a beam mixer between co-propagating edge channels, a still elusive building block of a recently proposed new class of quantum interferometers.

The AFM LAO lithography on GaMnAs layers

We prepared constrictions on ferromagnetic GaMnAs layer by the local anodic oxidation (LAO) using the atomic force microscope (AFM). These oxide lines, produced by the negatively biased AFM tip, formed the electrical barrier to the conducting holes in the layer. The constricted samples were characterized at low temperature (12 K). They showed magnetoresistance effect specific for nanoconstrictions during in-plane magnetic field sweep in both polarities for the different mutual orientation of magnetic field and current. The LAO appears to become a useful patterning technique for research of ferromagnetic semiconductor nanostructures. Further optimization of LAO parameters for reaching better homogeneity of the oxide lines is needed.

Fabrication of Nanostructure on a Polymer Film Using Atomic Force Microscope

SPM based lithographic techniques have been developed to pattern various substrates such as metals, semiconductors, and organic/polymer films due to its simplicity and high spatial precision nanostructure. Fabrication of nanostructure using polymeric materials is a key technique for the development of nanodevices. Here, we report the fabrication of nanostructures from polyacrylicacid (PAA) and polymethacrylicacid (PMAA) film on a silicon substrate using atomic force microscope (AFM). The formation of the nanopattern from the polymer film was studied using electrostatic nanolithography and the optimization of the conditions for nanopatterning of the polymer film was investigated with respect to the applied potential and translational speed of the AFM tip. The nanostructure of size 28 nm was created using the biased AFM tip on the PMAA film coated on Si(100) substrate and found that this method is a direct and reliable method to produce uniform nanostructures on a polymer film.

Ferroelectric domain phenomena and microdomain engineering in BaMgF4 single crystal

Ferroelectric domain phenomena and periodic domain patterning of orthorhombic as-grown BaMgF4 single crystal were investigated in the present study. An isolated ferroelectric domain shows a hexagonal form and exhibits a high domain-wall anisotropy. Periodic domain patterning was demonstrated on the polar surface by a periodic macropoling technique. In-plane polarization switching-induced microdomain patterning was tailored by the lateral component of the inhomogeneous electric field through a biased atomic force microscope tip. Such in-plane domain switching behavior exhibits potential promise for periodic domain engineered phonon devices.

Micro–nano behaviour ofDMBI-PF6 ionic liquid nanocrystals: large and small-scale interfaces

Microelectrical mechanical systems (MEMS) devices coated with a thin film ofionic liquid showed significant improvement in wear life. These promisingtribological findings led to the current study of the micro–nano behaviourof an ionic liquid, 1,2-dimethyl–3-butyl imidazolium hexafluorophosphate(DMBI-PF6), when applied as a thin film on a polished silicon surface. The films remain as microdropletsin ambient conditions but undergo drastic changes when agitated by an atomic forcemicroscope (AFM) contact scan or touched with a biased AFM tip. The nanotribologicalcharacteristics including mobility, diffusion and scratch resistance are studied. In the case ofDMBI-PF6, it was observed that the imidazolium ion is a mobile phase whilePF6 is attached to the silicon surface. Crystallites are formed as a result of tip contactand depend on the tip bias and environmental conditions. Crystallization ofDMBI-PF6 from the liquid phase with no tip bias showed substantial variation in crystal shapecompared to those formed under the influence of an electric bias.

Cutting of multiwalled carbon nanotubes by a negative voltage tip of an atomic force microscope: A possible mechanism

Multiwalled carbon nanotubes (MWNT's) on Si(5 5 12) surfaces are demonstrated to be cut only by a negatively biased conducting tip of an atomic force microscope (AFM). By scanning with the AFM tip across a 30-nm-diam MWNT in contact mode, we could cut the MWNT only at a negative tip voltage below a threshold. As the tip-moving speed increased, the magnitude of the threshold voltage was increased. A graphite surface was etched in comparison by the same method. It was also etched only at a negative tip voltage below a threshold. As the magnitude of the bias voltage increased, the etch depth of the graphite surface increased exponentially to reach 7.9 nm, a thickness of 23 atomic layers of graphite, at a bias voltage of $\ensuremath{-}10\mathrm{V}.$ The etching current from the graphite surface to the negatively biased tip was found to follow the Fowler-Nordheim equation and attributed to field-emission electrons from the negatively biased tip. The etch depth of the graphite surface was also found to follow the bias voltage dependence of the Fowler-Nordheim equation. The graphite etching is thus found to be controlled by the field-emission current so that we may propose a cutting mechanism based on the field-emission current density of the Fowler-Nordheim equation: both the MWNT cutting and graphite etching encounter the same reaction where the activation energy is supplied by electrons that are field emitted from the negatively biased AFM tip.

Quantum dot with independently tunable tunneling barriers fabricated using an atomic force microscope

We fabricate independently tunable tunneling barriers on a GaAs/AlGaAs heterostructure using a conducting tip of an atomic force microscope (AFM). In a humid ambient, a negatively biased AFM tip induces an oxide beneath it, which oxide depletes a shallow two-dimensional electron gas (2DEG). We exploit the depletion property of the oxide and use it to electrically isolate different regions of the 2DEG. In particular, we manipulate the tip movement to generate two oxide patterns whose combined geometry defines two quantum point contacts, in series, with a quantum dot between them. Our device not only permits independent tuning of the transmission probability of each quantum point contact, but it also allows us to observe Coulomb-blockade oscillations when both contacts are tuned into the tunneling regime.

Surface oxygenation studies on ( [formula omitted])-oriented diamond using an atom beam source and local anodic oxidation

Surface oxidation studies on pre-deuterated (1 0 0)-oriented single crystal diamond have been performed by oxidizing the diamond surfaces macroscopically using an oxygen atomic beam source as well as microscopically using local anodic oxidation by atomic force microscope (AFM). Oxygen-deuterium exchange on diamond (1 0 0) was investigated by X-ray photoelectron spectroscopy, elastic recoil detection and time-of-flight SIMS. Exchange of pre-adsorbed D by atomic O is thermally activated, with almost complete exchange of surface D by atomic O at 300 °C. At higher oxidation temperatures, oxidation states which are chemically shifted from the C 1s bulk peak by 3.2 eV was observed together with a disordering of the diamond surface. Micron-scale, localized oxygenation of the diamond surface at room temperature could be achieved with a biased AFM tip where we confirmed that the modified areas show a lower secondary electron yield and higher oxygen content. In addition, the electronic structure of the oxygenated diamond surface (on-top (OT) and bridging model) has been investigated by calculating the layered-resolved partial density of states using first principles plane wave ab initio pseudopotential method within the local density functional theory. For the oxygen OT model, sharp features due to occupied surface states in the valence band and unoccupied surface states in the gap exist. The increase in emission intensity near the valence band edge for oxygenated diamond (1 0 0) was verified by ultraviolet photoelectron spectroscopy study.

Local modification of the thin YBa2Cu3O7−y microstrips by the voltage-biased atomic force microscope tip

The atomic force microscope (AFM) tip biased at around −15 V is found to be capable of locally modifying the entire thickness of 40-nm-thick semiconducting or superconducting YBa2Cu3O7−y microstrips in air. We show, using combined electrical and AFM measurements, that the local regions underneath the surface of the semiconducting or superconducting YBa2Cu3O7−y microstrips are transformed into either nonconducting or nonsuperconducting regions, respectively, upon applying the negatively biased AFM tip. The conductance of the nonsuperconducting regions is also found to be comparable to that of the superconducting regions before modification at 298 K.

Nanoscale imaging of domain dynamics and retention in ferroelectric thin films

We report results on the direct observation of the microscopic origins of backswitching in ferroelectric thin films. The piezoelectric response generated in the film by a biased atomic force microscope tip was used to obtain static and dynamic piezoelectric images of individual grains in a polycrystalline material. We demonstrate that polarization reversal occurs under no external field (i.e., loss of remanent polarization) via a dispersive continuous-time random walk process, identified by a stretched exponential decay of the remanent polarization.