Biased Atomic Force Microscope Research Articles

Local electrical properties and charging/discharging of CdSe/CdS core-shell nanoplatelets

Quantum confinement in two-dimensional semiconductor nanoplateletes (NPLs) is determined by their thickness which can be precisely controlled during the synthesis. As a result, NPLs have a very narrow luminescence spectrum and they can provide light sources with very high color purity. Switchable light sources needed for a wide range of applications require the dynamic control of the luminescence. One efficient approach for this purpose is direct charge injection into NPLs. In order to study charging/discharging processes and local electrical properties of CdSe/CdS core-shell NPLs as the model system, here we employed electrical methods based on atomic force microscopy (AFM). Simple and efficient procedures for “write/read/erase” operations are presented: charges are written by a biased AFM tip in contact with the NPLs, their charge state is read by Kelvin probe force or electric force microscopy, whereas injected charges are erased by inversely biased AFM tip. The amount of injected charges is well controlled by a magnitude, polarity and duration of the applied bias voltage, whereas the rate of subsequent spontaneous charge relaxation is dominantly determined by ambient humidity.

Contact electrification and charge decay on polyester fibres: A KPFM study

Surface contact electrification experiments have been performed on insulating polyester fibres in contact with a conductive fibre using biased atomic force microscopy tips. With positive tip bias, charge spots have been deposited in a reproducible manner. The density of deposited charges on the fibre surface was observed by Kelvin probe force microscopy as a function of time, charging position and relative humidity. Two main charge dissipation mechanisms were identified with different importance depending on the relative humidity value.

Nanoscale mapping of dielectric properties based on surface adhesion force measurements

The detection of local dielectric properties is of great importance in a wide variety of scientific studies and applications. Here, we report a novel method for the characterization of local dielectric distributions based on surface adhesion mapping by atomic force microscopy (AFM). The two-dimensional (2D) materials graphene oxide (GO), and partially reduced graphene oxide (RGO), which have similar thicknesses but large differences in their dielectric properties, were studied as model systems. Through direct imaging of the samples with a biased AFM tip in PeakForce Quantitative Nano-Mechanics (PF-QNM) mode, the local dielectric properties of GO and RGO were revealed by mapping their surface adhesion forces. Thus, GO and RGO could be conveniently differentiated. This method provides a simple and general approach for the fast characterization of the local dielectric properties of graphene-based materials and will further facilitate their applications in energy generation and storage devices.

The jump-into-contact effect in biased AFM probes on dielectric films and its application to quantify the dielectric permittivity of thin layers

The jump-into-contact (JIC) phenomenon in biased atomic force microscopy (AFM) probes on dielectric films is studied. The influence of the film thickness on the position at which the AFM tip collapses irreversibly into the sample surface was theoretically analyzed using a widely accepted analytical expression of the probe–sample electrostatic interaction force. It was demonstrated that for relatively high values of voltage (V > 10–20 V) applied between the probe and the substrate the cantilever deflection at the JIC is independent of the dielectric film thickness for thin–ultrathin layers (h < 10–50 nm). Under the same conditions the z–piezo distance at the JIC follows approximately a linear behavior with the film thickness. Based on this effect an empirical model was formulated to estimate the dielectric permittivity of thin/ultrathin dielectric films using the jump-into-contact distance. The procedure was successfully applied on thin PVD–SiO2 films, obtaining good agreement with a dielectric constant value previously reported for the same material.

Selective and directed growth of silicon nanowires by tip-enhanced local electric field

We present a method to trigger highly selective and directed growth of individual silicon nanowires based on an electrically biased atomic force microscope (AFM) tip. The biased tip affects the nanowire growth behavior right from the initial stage. In particular, the locally intensified electric field at the AFM tip apex assists the dissociation of the precursor gas molecules and exerts augmented electrostatic force on the catalyst/NW assembly. Therefore, the electrically biased tip can be a candidate tool for the direct synthesis/integration of semiconductor nanomaterials on temperature-sensitive substrates.

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.

Revealing energy level structure of individual quantum dots by tunneling rate measured by single-electron sensitive electrostatic force spectroscopy.

We present theoretical and experimental studies of the effect of the density of states of a quantum dot (QD) on the rate of single-electron tunneling that can be directly measured by electrostatic force microscopy (e-EFM) experiments. In e-EFM, the motion of a biased atomic force microscope cantilever tip modulates the charge state of a QD in the Coulomb blockade regime. The charge dynamics of the dot, which is detected through its back-action on the capacitavely coupled cantilever, depends on the tunneling rate of the QD to a back-electrode. The density of states of the QD can therefore be measured through its effect on the energy dependence of tunneling rate. We present experimental data on individual 5 nm colloidal gold nanoparticles that exhibit a near continuous density of state at 77 K. In contrast, our analysis of already published data on self-assembled InAs QDs at 4 K clearly reveals discrete degenerate energy levels.

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.

A General Approach for Fast Detection of Charge Carrier Type and Conductivity Difference in Nanoscale Materials

A general method using a biased atomic force microscopy tip that allows a qualitative, fast, and reliable determination of key electronic properties such as metallic, n-, or p-doped characteristics has been reported for the first time. This method eliminates the detrimental effect of contact in the traditional transport measurement and is much simpler than the common-electrostatic force microscopy detection method, thus providing a powerful tool for fast characterizations of nanomaterials.

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.

Local Scanning Probe Polymerization of an Organic Monolayer Covalently Grafted on Silicon

The possibility of lateral extension of conjugation within a covalently grafted molecular layer by a scanning probe-based method was tested. A molecular layer derived from ω-(N-pyrrolyl)propanol was formed on n-type Si(111) surface. Application of large sample biases greater than ±4 V during conductive atomic force microscope (AFM) scans under vacuum resulted in changes of mechanical and electrical characteristics of the molecular layer: the tip-sample conductance was increased greatly, the friction was reduced significantly, and the surface potential of the scanned area was increased. The reduction in friction could be attributed to molecular linking formed within the layer. The increased conductance suggested extended conjugation among the pyrrolyl end groups. Therefore, it was inferred that the biased AFM scan successfully induced local polymerization/oligomerization within the covalently grafted molecular layer.

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.

New insights into reliability of electrostatic capacitive RF MEMS switches

Among other reliability concerns, the dielectric charging is considered the major failure mechanism which hinders the commercialization of electrostatic capacitive radio frequency micro-electro-mechanical systems (RF MEMS) switches. In this study, Kelvin probe force microscopy (KPFM) surface potential measurements have been employed to study this phenomenon. Several novel KPFM-based characterization methods have been proposed to investigate the charging in bare dielectric films, metal–insulator–metal (MIM) capacitors, and MEMS switches, and the results from these methods have been correlated. The used dielectric material is plasma-enhanced chemical vapor deposition (PECVD) silicon nitride. The SiNxfilms have been charged by using a biased atomic force microscope (AFM) tip or by electrically stressing MIM capacitors and MEMS switches. The influence of several parameters on the dielectric charging has been studied: dielectric film thickness, deposition conditions, and under layers. Fourier transform infra-red (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS) material characterization techniques have been used to determine the chemical bonds and compositions, respectively, of the SiNxfilms. The data from the physical material characterization have been correlated to the KPFM results. The study provides an accurate understanding of the charging/discharging processes in dielectric films implemented in electrostatic MEMS devices.

Charge storage in mesoscopic graphitic islands fabricated using AFM bias lithography

Electrochemical oxidation and etching of highly oriented pyrolytic graphite (HOPG) hasbeen achieved using biased atomic force microscopy (AFM) lithography, allowing patternsof varying complexity to be written into the top layers of HOPG. The graphiticoxidation process and the trench geometry after writing were monitored usingintermittent contact mode AFM. Electrostatic force microscopy reveals that the isolatedmesoscopic islands formed during the AFM lithography process become positivelycharged, suggesting that they are laterally isolated from the surrounding HOPGsubstrate. The electrical transport studies of these laterally isolated finite-layergraphitic islands enable detailed characterization of electrical conduction along thec-direction and reveal an unexpected stability of the chargedstate. Utilizing conducting-atomic force microscopy, the measuredI(V) characteristics revealed significant non-linearities. Micro-Raman studies confirmthe presence of oxy functional groups formed during the lithography process.

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.

Contact Angle Analysis During the Electro‐oxidation of Self‐Assembled Monolayers Formed by n‐Octadecyltrichlorosilane

AbstractThe electrochemical oxidation process of self‐assembled monolayers formed by n‐octadecyltrichlorosilane (OTS) molecules on silicon wafers has been studied in a droplet of water by means of in situ water contact angle measurements. The application of different bias voltages between the substrate and a counter electrode placed into the droplet resulted in changes of the chemical nature of the monolayer, which yielded a significant alteration of the surfaces properties. Due to the changes of the wetting properties of the monolayer during the electro‐oxidation process a change in the contact angles of the water droplet is concomitantly observed. This allows the in situ monitoring of the electro‐oxidation process for large modified areas of several millimeters in diameter. The chosen approach represents an easy way to screen the major parameters that influence the oxidation process. Afterwards, the oxidized regions are characterized by Fourier‐transform infrared (FT‐IR) spectroscopy, X‐ray photoelectron spectroscopy (XPS) measurements, and atomic force microscopy (AFM) investigations to obtain more information about the electro‐oxidation process. The observations are correlated to experimental results obtained for oxidations performed on a smaller dimension range in the water meniscus of a conductive, biased AFM tip. A good correlation of the results in the different dimension ranges could be found.

Electrocondensation and evaporation of attoliter water droplets: Direct visualization using atomic force microscopy

Working with a biased atomic force microscope (AFM) tip in the tapping mode under ambient atmosphere, attoliter (10−18 L) water droplet patterns have been generated on a patterned carbonaceous surface. This is essentially electrocondensation of water leading to charged droplets, as evidenced from electrostatic force microscopy measurements. The droplets are unusual in that they exhibit a highly corrugated surface and evaporate rather slowly, taking several tens of minutes.

Submicron gold structures formed by atomic force microscopy lithography on thin films of poly(methyl methacrylate)

Thin films of poly(methyl methacrylate) (PMMA) (25 nm) deposited on gold coated glass substrates have been locally modified by exposure to biased atomic force microscopy (AFM) tips. Constant exposure currents in the range of 0.5–0.65 nA leads to removal of PMMA both by direct ablation and enhanced solubility in a developer solution. Possible mechanisms causing such modifications have been discussed. Submicron Au structures were formed by combining AFM lithography, involving localized removal of PMMA, with sputter deposition of gold and a lift-off process.