Conductive AFM Tip Research Articles

Selective formation of novel fluorescent aggregates of rhodamine dyes: contribution of surface SiO2 layer and underlying Si layer

AbstractRhodamine (Rh) 6G aggregates were formed on a silicon substrate with a varying thickness surface SiO2 layer utilizing a dewetting process of Rh ethanol solution. Sufficient thickness of 30 nm SiO2 layer gave rise to fluorescent aggregates selectively, similar to those formed on a hydrophilic glass surface. Annealing at 200 oC forced a phase transition of those aggregates to fluorescence quenched state, which was also common with those formed on a hydrophilic glass surface. In contrast, sufficiently thin, e.g. naturally formed at ambient condition, 2 nm thick SiO2 layer gave rise to quenched aggregates, that was spectroscopically identical with the annealed aggregates formed on thick SiO2 layer. Whereas the quenching on sufficiently thin SiO2 layer may be partially due to a nonradiative energy transfer to underlying Si, the majority of the quenching was attributed to specific Rh molecular ordering that is thermodynamically stable than the quasi‐stable fluorescent Rh aggregates. This quasi‐stable aggregates were formed only on sufficiently thick SiO2 layer or on hydrophilic glass substrate. This finding will shed light on the disputed discrepancy of fluorescence quenching of Rh dyes deposited on SiO2 nanostripes that were formed on a Si substrate by local anodic oxidation using a conductive AFM tip. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Thermionic emission from the 2DEG assisted by image-charge-induced barrier lowering in AlInN/AlN/GaN heterostructures

The effect of image charges on current transport mechanisms investigated at the nanoscale in Al1−xInxN/GaN heterostructures was studied. Current–voltage (I–V) measurements were performed locally using a conductive AFM-tip as a nanoprobe and the conduction mechanism was modeled to explain the observed behavior. This model suggests that current transport is controlled by thermionic emission (TE) of the two-dimensional electron gas (2DEG) across the potential barrier at the heterointerface, where the image charges generated by the 2DEG induce a barrier lowering at the Al1−xInxN/GaN interface, enhancing electron transport. This barrier lowering depends on the 2DEG characteristics, such as 2DEG density n2D, first subband energy E0 and the average distance x0 of the 2DEG from the interface. By fitting the experimental I–V curves with the present model the 2DEG density was evaluated. The obtained results were in very good agreement with the Hall measurements.

Nanoscale Probing of High Photovoltages at 109° Domain Walls

We report the direct observation of nanoscale photovoltaic current-voltage properties associated with directional steps of the electrostatic potential at 109° domain walls in ferroelectric BiFeO3 thin films with highly ordered ferroelectric stripe domains. The unidirectional photovoltaic effect along the net-in-plane polarization direction is shown to exhibit anomalies at the micro- and nanoscale due to local differences in mesoscopic domain structure. Very high initial photovoltages for small electrode spacings are observed using a conductive AFM tip as one of the electrodes in photovoltaic I-V measurements.

Automated sub-100 nm local anodic oxidation (LAO)-directed nanopatterning of organic monolayer-modified silicon surfaces

The fabrication of silicon oxide nanopatterns using the local anodic oxidation (LAO) by a conducting AFM tip of different organic monolayer (decyl, dodecyl, hexadecyl and undecanoic acid)-modified Si(111) surfaces is reported. It is demonstrated that the threshold bias voltage of oxide formation increases with the monolayer thickness and decreases when acid end groups are used instead of methyl. The automatization of the LAO process has allowed the perfect replication of vectorized complex drawings (e.g. compass card and longship) on silicon using the undecanoic acid monolayer as the model molecular resist. Under optimized experimental conditions (namely, bias voltage of 8 V applied to the surface and writing rate of 1 μm s−1), highly reproducible and uniform 2–3 nm thick oxide patterns have been electrogenerated with a minimum lateral resolution of 80 nm. Interestingly, the silicon oxide nanopatterns have been selectively dissolved in diluted hydrogen fluoride solution to generate potentially reactive hydrogenated areas. These hydrogenated sites have then been used for the electroless deposition of gold nanoparticles by galvanic displacement. This approach was found to be convenient and fast to metallize the patterns initially produced by LAO. The average diameter of deposited gold particles decreased from 20 to 2 nm with decreasing the gold salt concentration from 100 to 1 μM.

Nanoscale electrochemical measurements on a lithium-ion conducting glass ceramic: In-situ monitoring of the lithium particle growth

We performed nanoscopic cyclic voltammetry and chronoamperometry on the Ohara lithium ion conductive glass ceramic. When the critical bias voltage was exceeded, metallic lithium particles were deposited irreversibly under the conductive AFM tip acting as working electrode. Due to the huge area difference between working electrode and counter electrode, a reference electrode was not required. We present an in-situ method for studying the particle growth mechanism by simultaneously monitoring the reduction current and the particle height. We found good agreement between the topographically determined volume of the lithium particles and the volume expected from the charge flow. The results for time dependence of the particle growth can be used for controlling the vertical and lateral dimensions of the particles.

Nanoscale Observation of the Distribution Polarization in Lithium Niobate Thin Films

ABSTRACTIn this work, an Atomic Force Microscope in the so-called Piezoresponse mode and Kelvin mode is used to image the grains, ferroelectric domains and surface potential in lithium niobate thin films. A RF magnetron sputter system was used to deposit LiNbO3 thin films on (100)-oriented Si substrates with SiO2 layer. The surface of the sample shows small grains which diameter ranges from 70 nm to 150 nm and roughness is less than 13 nm. Using the electric field from a biased conducting AFM tip, we show that possible to form and subsequently to visualize ferroelectric state. Also, we report surface charge retention on ferroelectric thin films by Kelvin probe microscope in comparison with the piezoresponse signal.

Quantitative nanoscopic impedance measurements on silver-ion conducting glasses using atomic force microscopy combined with impedance spectroscopy

Nanoscopic impedance measurements were carried out on silver ion conducting glasses by coupling an impedance spectrometer with an atomic force microscope. When ac voltages were applied to a conducting AFM tip being in contact with the glass surface, silver nanoparticles were formed during the cathodic half cycle, which were not completely reoxidized in the anodic half cycle. We describe two protocols allowing for a controlled particle growth. The electrochemical oxidation/reduction processes led to low tip/sample interfacial impedances, and the formed silver particles acted as nanoelectrodes sensing the spreading resistance of the glass below the particles. We made a quantitative check of the spreading resistance formula under the assumption that spreading of the electric field is governed by the lateral diameter of the particles and found good agreement between the mean value of the local conductivities obtained at different tip positions and the macroscopic conductivity.

Current-Voltage Characteristics of Side-Gated Silicon Nanowire Transistor Fabricated by AFM Lithography

Silicon nanowire transistor (SiNWT) was fabricated by using a silicon nanowire as a channel which directly connected to the source (S) and drain (D). In this work, a side gate (G) formation was used to develop a transistor structure. AFM lithography was performed to create the nanoscale oxide patterns via local anodic oxidation (LAO) mechanism. A conductive AFM tip was used to grow localized oxide layer on the surface of silicon on insulator (SOI) substrate by the application of voltage between tip and substrate. Other parameters that will influence the patterning process such as tip writing speed, relative air humidity, anodization time and substrate orientation were controlled. The patterned structure was etched with tetramethylammonium hydroxide (TMAH) and hydrogen fluoride (HF) acid to remove the uncovered silicon layer and silicon oxide mask patterns, respectively. The surface topography and dimension of the fabricated SiNWT was observed under AFM. Obtained results for the channel thickness, channel length and the distance between the channel and side gate are 32.92 nm, 7.63 µm and 108.07 nm, respectively. Meanwhile, the I-V characteristics of fabricated SiNWT measured at positive gate voltages are similar to p-type FET characteristics.

Self-aligned nucleation of gold onto templates with a nano-scale precision fabricated by scanning probe lithography

Nanostructures consisting of concentric Au circles with a nm-scale line thickness have been fabricated on silicon (Si) substrates based on electroless plating and by the use of scanning probe microscope as a lithographic tool. First, a micro-template was fabricated on a hexadecyl self-assembled monolayer (HD-SAM) covalently bonded to a Si(1 1 1) substrate by scanning probe anodization lithography in which the HD-SAM was locally degraded and the substrate Si was anodized along a trace of an conductive AFM-tip biased negatively to the grounded Si substrate. Anodic Si oxide formed by this local anodization process was then etched with HF in order to expose the Si surface under the anodic oxide layer. Consequently, a Si nanopattern surrounded with the HD-SAM identical to the anodic oxide pattern was formed. This Si nanopattern served as a template for fabricating a Au nanostructure by means of electroless plating. Due to the galvanic displacement of surface Si atoms with Au complex ions in a plating bath, Au started to nucleate on the exposed Si surface, while no nuclei was formed on the surrounding HD-SAM. Consequently, a Au nanostructure was formed through further Au deposition on the Au nuclei. The fabricated Au nanostructures are promising for applications emerging plasmonic functions.

Electrical characterization of epitaxial FeSi2 nanowire on Si (110) by conductive-atomic force microscopy

We used conductive-atomic force microscopy (c-AFM) for electrical characterization of self-assembled epitaxial iron silicide nanowires (NWs) on Si (110). The NWs, 6 nm high by 10 nm wide and several micrometers long, were partially covered by a macro-gold-pad as one electrode. Another electrode is the conductive AFM tip. The resistance of a single FeSi2 NW was measured to be 29.7 kΩ, corresponding to a resistivity of 150 ± 30 μΩ·cm. A Schottky barrier formed between NWs and silicon substrate was clearly demonstrated, which offers electrical isolation for NWs. An equivalent circuit model based on the Schottky barrier was proposed and was correlated with measurement results. This simple electrical characterization approach may find wide applications for various one-dimensional nanostructures.

Sub-10-nm patterning of oligo(ethylene glycol) monolayers on silicon surfaces via localoxidation using a conductive atomic force microscope

We demonstrate that local oxidation using a conductive atomic force microscope (c-AFM)can achieve patterning of sub-10-nm spots on protein-resistant oligo(ethyleneglycol)-terminated alkyl monolayers on silicon substrates. Such a high resolution ofnanopatterning with a c-AFM on organic thin films was realized for the firsttime by applying ultrashort (50–100 ns) voltage pulses to the silicon substraterelative to the conducting AFM tip. Furthermore, the nanopatterning can becontrolled at the surface of the hydrophilic monolayer without oxidizing the siliconinterface.

ZnO−ZnS Heterojunction and ZnS Nanowire Arrays for Electricity Generation

Vertically aligned ZnO-ZnS heterojunction nanowire (NW) arrays were synthesized by thermal evaporation in a tube furnace under controlled conditions. Both ZnO and ZnS are of wurtzite structure, and the axial heterojunctions are formed by epitaxial growth of ZnO on ZnS with an orientation relationship of [0001](ZnO)//[0001](ZnS). Vertical ZnS NW arrays have been obtained by selectively etching ZnO-ZnS NW arrays. Cathodoluminescence measurements of ZnO-ZnS NW arrays and ZnS NW arrays show emissions at 509 and 547 nm, respectively. Both types of aligned NW arrays have been applied to convert mechanical energy into electricity when they are deflected by a conductive AFM tip in contact mode. The received results are explained by the mechanism proposed for nanogenerator.

PEG-Assisted Fabrication of Single-Crystalline CuI Nanosheets: A General Route to Two-Dimensional Nanostructured Materials

CuI single-crystalline nanosheets have been prepared for the first time via a PEG-assisted aqueous solution route at room temperature. The thickness and in-plane size of the nanosheets were ca. 60−80 nm and several micrometers, respectively. The two basal surfaces of these nanosheets were (111) planes. The phase transformation temperature and the melting point decreased 8 and 12 °C compared with those of the bulk CuI, respectively. The resistance of a single CuI nanosheet was measured by using a conductive AFM tip method, and a high conductivity of 1.996 × 10-2 Ω·cm and a photoconduction phenomenon were observed. It is found that the coexistence of PEG600 and SDBS was vital to the formation of nanosheets. This simple route could be employed to synthesize more 2D nanostructures as a general process.

Fabrication of gold nanoelectrodes based on nanolithography electrochemically through a conductive AFM tip

Gold nanoelectrodes were fabricated by approach of combining surface self-assembly with nanolithography electrochemically through a conductive AFM tip. The controllable structure with width and distance in nanometer as a template was constructed by nanolithography patterning process, which was accomplished by a conductive AFM tip on certain highly ordered long-tail organosilane monolayers. Then through adsorption of Cd2+ and exposure of the Cd2+-loaded surface to gaseous H2S, CdS nanowires were generated in a template-controlled self-assembly process. Finally, metallic gold nanowires were conversed from CdS nanowires by treatment with the aqueous solution of HAuCl4 via a redox chemical process, which had good conductivity proved by C-AFM.

Nano-Patterning in Polymeric Materials and Biological Objects Using Atomic Force Microscopy Electrostatic Nanolithography

In this article we are concerned with several aspects related to atomic force microscopy electrostatic nanolithography (AFMEN). AFMEN technique is based on manipulation of nano-amounts of dielectric materials in strong 10 8 - 10 10 Vm-1 electric field. In polymer films of different physical-chemical properties AFMEN produces erasable nanostructures that are 10-300 nm in diameter and 0.5-20 nm in height through Joule heating of polymeric molecules between conductive AFM tip and substrate followed by electrostatic attraction of the softened polymer towards the tip. We discuss AFMEN for potential data storage applications in polymeric materials. Additionally, it is demonstrated that AFMEN manipulates bundles of Wiseana Iridovirus and produces noticeable changes in capsids composing individual virions. Keywords: nano-patterning techniques, scanning probe microscopies (SPMs), scanning tunneling microscopy (STM), self-assembled monolayers, Columbic force, DC voltage pulse, electrization, folded protein helices

Conductance fluctuation and degeneracy in nanocontact between a conductive AFM tip and a granular surface under small-load conditions

Abstract Conductive-tip atomic force microscope (c-AFM) has been extensively used in measuring electrical properties of surface nanostructures, but the electrical conduction in c-AFM tip–sample contacts in nanometer scale is not well understood. In the present work, we experimentally investigated the electrical properties of the nanocontact between a W2C-coated c-AFM tip and granular gold film under small-load (∼5 nN) at ambient air conditions. We found that under a constant bias voltage (10 V), the electrical current passing through the tip–sample junction at fixed location of sample surface dramatically fluctuated and degenerated. By quantitatively estimating the mechanical and electrical aspects of the nanocontact, we explained the observed phenomena as mechanical instabilities, electron tunneling transport and atomic rearrangements at the contact junction. We think that our results are important for the realistic application of c-AFM in nanoelectronic measurement.

Fabrication of gold nanoelectrodes based on nanolithography electrochemically through a conductive AFM tip

Fabrication of gold nanoelectrodes based on nanolithography electrochemically through a conductive AFM tip

Covalent integration of pyrrolyl units with modified monocrystalline silicon surfaces for macroscale and sub-200 nm-scale localized electropolymerization reactions

Macroscale and localized electrochemical growths of polypyrrole deposits have been performed on monocrystalline Si(111) surfaces modified by pyrrole-terminated organic monolayers. The first approach proposed to induce localized electropolymerization reaction consisted of the high dilution of pyrrole-terminated chains with inert decyl chains in order to have a large separation between the attached monomer units. Unfortunately, this method failed to produce polypyrrole features with precision and control of the structure geometry. More successful results were obtained with the electrochemical dip-pen nanolithography (DPN). A pyrrole-coated conducting AFM tip was brought into contact with a pyrrole-terminated silicon surface and a positive bias of +1.0 V was then applied to the surface for a given period of time. Along this lithographic progress, polypyrrole dots with diameters in the range of 75–200 nm and sub-200 nm-wide lines could be electrogenerated.

Molecular electron transfer of protein junctions characterised by conducting atomic force microscopy

The transport characteristics of the blue copper metalloprotein, azurin, have been characterised by conducting atomic force microscopy (C-AFM) at molecular level. Tunnel junctions have been constructed by sandwiching chemisorbed protein molecules between a conducting AFM tip and a planar conducting substrate. Asymmetric current curves with respect to the polarity of the bias ( I– V) have been observed. The modulation of I– V behaviour with compressional force has been examined and is described by a modified Simmons model within which both tunnel distance (protein dimensions) and tunnel barrier are modulated. The modified Simmons formula, which considered unequal Fermi level shifts on two electrodes as being responsible for the asymmetric I– V curves, accurately describes the behaviour observed.

Resonance Tunneling Diode Structures on CdTe Nanowires Made by Conductive AFM

Variation of band gap across the nanowire length would be an exceptionally attractive property for the fabrication of on-nanowire devices such as resonance tunneling diodes (RTD). Band gap variation can be achieved by selective thinning of semiconductor wires by scanning probe lithography (SPL) technique. The external bias applied to a conductive AFM tip during scanning of CdTe nanowires was chosen so as to exceed the threshold of electric field-assisted evaporation of CdTe, estimated to be 5.5 V. Relatively high external voltages of 10−11 V cause fast and complete disintegration of a nanowire portion under the tip. In this way the nanowire can be cut to a desired length. Selection of a voltage between 5.5 and 10 V allows one to control the speed of CdTe evaporation. Thus, one can modulate the thickness of the semiconductor with angstrom scale precision along the nanowire length. Smaller diameter of the nanowire results in increase of quantum confinement in selected areas. The double barrier quantum well valence band profile necessary for the manufacturing of RTD Esaki diodes was demonstrated.