Conductive AFM Tip Research Articles

Grafted polythiophene pendent polymer brushes and their electronanopatterning

• Nanolithography and patterning using polymer brushes with terthiophene pendents. • Terthiophene-containing chain transfer agent macroinitiator. • Nanolithography and patterning were done using conducting atomic force microscopy. • Nanopattern properties depend on the applied bias voltage and scan rate. Electrochemical nanolithography using grafted polymer brushes with terthiophene (3T) pendent was investigated. The film was fabricated by surface electropolymerization of a reversible addition−fragmentation chain-transfer agent macroinitiator containing a 3T pendant molecule and the subsequent polymer brush growth from a 3T functionalized methacrylate monomer (3T-Methacrylate). Electrochemical nanolithography and patterning were done using conducting atomic force microscopy (C-AFM) enabling precise nanopattern fabrication. In contrast to direct nanowriting, dip pen nanolithography, and nanolithographic additive manufacturing, the electrochemical nanolithography can be readily facilitated by applying a bias voltage between a conductive AFM tip and the grafted polymer brush film at ambient conditions and without ink transfer. The height and electrical resistance of the nanopatterns were dependent on the writing parameters (i.e. applied bias voltage and scan rate).

Defect states contributed nanoscale contact electrification at ZnO nanowires packed film surfaces

Efficient conversion of mechanical energy in our surrounding environment into electric power has become a promising strategy for meeting the ever-increasing energy consumption of small and distributed electronics. The contact-electrification-based triboelectric nanogenerators are one of the emerging devices to achieve such energy conversion. However, conventional contact electrifications between two insulators are limited by their low current density and alternating current output. Here we report a nanoscale contact electrification induced direct current output based on the flow of electrons from the defect states of the ZnO nanowires-packed film to the contact sliding conductive AFM tip. Combining experimental materials characterization and density functional theory (DFT) calculations, the direct current output is closely related to the concentration of oxygen vacancy defect states on the surface of ZnO nanowires: the higher the oxygen vacancy concentration, the higher the current output. Under optimized conditions, we obtain an ultrahigh current density of ~108 A m-2, which is several orders of magnitude higher than that of the conventional contact electrification and other effects. This work provides a new route of utilizing defect states contributed contact electrification for realizing nanoscale mechanical energy scavenging.

Fabrication of SiNWs-FET Nanostructure Via Atomic Force Microscopy Lithography

The unique electrical properties of silicon nanowires (SiNWs) is one of the reasons it become an attractive transducer for biosensor nowadays. Positive (holes) and negative (electron) charge carriers from SiNWs can simply interact with either positive or negative charge of sensing target. In this paper, we have studied the fabrication of silicon nanowires field effect transistor (SiNWs-FET) nanostructure patterned on 15 Ω resistivity of p-type silicon on insulator (SOI) wafer fabricated via atomic force microscopy lithography technique. To fabricate SiNWs-FET nanostructure, a conductive AFM tip, Cr/Pt cantilever tip, was used then various value of applied voltage, writing speed and relative humidity were studied. Subsequent, followed by wet etching processes, admixture of tetramethylammonium hydroxide (TMAH) and isopropyl alcohol (IPA) were used to remove the undesired of silicon layer and diluted hydrofluoric acid (HF) was used to remove the oxide layer. From the results, it shows that, cantilever tip at 9 V with 0.4 μm/s writing speed and relative humidity between 55% - 60% gives the best formation of silicon oxide to fabricate SiNWs-FET nanostructure.

Probing the origins of electroresistance switching behavior in ferroelectric thin films

Ferroelectric thin films have been systematically investigated via scanning probe microscopy in recent years. Research indicates that the surface potential is the combined result of injected and polarization charges. The relationships between surface potentials and the two types of charges are usually investigated via scanning Kelvin probe microscopy (SKPM). Typically, SKPM investigations indicate that the surface charge distribution is dominated by injected charges trapped during poling using a conductive AFM tip rather than by polarization charges. The presence of injected charges leads to controversy concerning the origins of resistive switching behavior. In this study, relaxation of injected charges was observed during an optimized thermal treatment. This caused polarization charges to dominate over injected charges. Different electroresistance switching characterizations were observed via conductive atomic force microscopy (C-AFM) of injected and polarization charge-dominated films. Our research extends the methods of distinguishing whether electroresistance switching behavior is driven by charge trapping/detrapping or ferroelectric polarization. This provides an effective approach to classifying the origins of electroresistance switching in ferroelectric thin films by combining piezoelectric force microscopy, SKPM, and C-AFM.

Reconfigurable edge-state engineering in graphene using LaAlO3/SrTiO3 nanostructures

The properties of graphene depend sensitively on doping with respect to the charge-neutrality point (CNP). Tuning the CNP usually requires electrical gating or chemical doping. Here, we describe a technique to reversibly control the CNP in graphene with nanoscale precision, utilizing LaAlO3/SrTiO3 (LAO/STO) heterostructures and conductive atomic force microscope (c-AFM) lithography. The local electron density and resulting conductivity of the LAO/STO interface can be patterned with a conductive AFM tip [Cen et al., Nat. Mater. 7, 298 (2008)] and placed within two nanometers of an active graphene device [Huang et al., APL Mater. 3, 062502 (2015)]. The proximal LAO/STO nanostructures shift the position of graphene CNP by ∼1012 cm−2 and are also gateable. Here, we use this effect to create reconfigurable edge states in graphene, which are probed using the quantum Hall effect. Quantized resistance plateaus at h/e2 and h/3e2 are observed in a split Hall device, demonstrating edge transport along the c-AFM written edge that depends on the polarity of both the magnetic field and direction of currents. This technique can be readily extended to other device geometries.

Metallic-semiconducting junctions create sensing hot-spots in carbon nanotube FET aptasensors near percolation

Easily fabricated random network carbon nanotube field-effect transistors (CNT-FETs) have benefitted from improved separation techniques to deliver CNTs with current formulations providing at least 99% semiconducting tube content. Amongst the most promising applications of this device platform are electronic biosensors, where the network conduction is affected through tethered probes such as aptamers which act as molecular scale electrostatic gates. However, the prevailing assumption that these biosensor devices would be optimized if metallic tubes were entirely eliminated has not been examined. Here, we show that metallic-semiconducting junctions in aptasensors are sensing hotspots and that their impact on sensing is heightened by the CNT network's proximity to percolation. First, we use a biased conducting AFM tip to gate a CNT-FET at the nanoscale and demonstrate that the strongest device response occurs when gating at metallic-semiconducting junctions. Second, we resolve the target sensitivity of an aptasensor as a function of tube density and show heightened sensitivity at densities close to the percolation threshold. We find the strongest sensing response where the 1% of metallic tubes generate a high density of metallic-semiconducting junctions but cannot form a percolated metallic path across the network. These findings highlight the critical role of metallic tubes in CNT-FET biosensor devices and demonstrate that network composition is an important variable to boost the performance of electronic biosensors.

Metal whisker growth induced by localized, high-intensity DC electric fields

ABSTRACTIn this work, a very high, locally applied electric field was used to induce whisker nucleation on an Sn film. The field was generated by using a conductive AFM tip and applying a voltage bias between the sample and the conductive cantilever. The tip-sample separation distance was thus controllable, and any dielectric breakdown could be avoided. At locations where the AFM tip was positioned for an extended period, minuscule whiskers were observed, whose growth direction matched vertical orientation of the field.

Transport properties of Si based nanocrystalline films investigated by c-AFM

SiOxNy is an innovative material that has recently attracted a lot of attention in different and new applications, ranging from photovoltaics, conductive oxide, carbon capture; nevertheless, due to its complex and multiphase nature, the understanding of its electrical properties is still ongoing. In this framework, the present manuscript presents the investigation of electrical transport properties of nanocrystalline (nc-) SiOxNy. In fact, non-stoichiometric nc-SiOxNy films deposited by Plasma Enhanced Chemical Vapor Deposition have been studied by conductive Atomic Force Microscopy (AFM). The analyses of samples subjected to different thermal treatments conditions and the comparison with nc-Si:H films have allowed us to clarify the role of crystallization and O content on the local conductivity of the layers. We show that the annealing treatment promotes an enhancement of conductance, a redistribution of the conductive grains in the layers and the activation of B doping. Current-voltage characteristics locally performed using the conductive AFM-tip as a nanoprobe have been modelled with thermionic emission transport mechanism.

Assessment of copper surface coverage with corrosion inhibitor using AFM-based local electrical measurements

ABSTRACTThe paper presents a new method of assessment of metal surface coverage with corrosion inhibitor and thus of inhibitor protective performance. It is based on the atomic force microscopy measurement performed in a contact mode. Apart from topography images the proposed approach allows acquisition of local DC maps and local electrical impedance spectra via application of DC bias voltage or AC perturbation signal between the conductive AFM tip and the substrate. Potentialities of this technique in inhibitor performance monitoring were illustrated on the example of copper/benzotriazole system exposed to elevated humidity environment.

Addressable Direct-Write Nanoscale Filament Formation and Dissolution by Nanoparticle-Mediated Bipolar Electrochemistry.

Nanoscale conductive filaments, usually associated with resistive memory or memristor technology, may also be used for chemical sensing and nanophotonic applications; however, realistic implementation of the technology requires precise knowledge of the conditions that control the formation and dissolution of filaments. Here we describe and characterize an addressable direct-write nanoelectrochemical approach to achieve repeatable formation/dissolution of Ag filaments across a ∼100 nm poly(ethylene oxide) (PEO) film containing either Ag+ alone or Ag+ together with 50 nm Ag-nanoparticles acting as bipolar electrodes. Using a conductive AFM tip, formation occurs when the PEO film is subjected to a forward bias, and dissolution occurs under reverse bias. Formation-dissolution kinetics were studied for three film compositions: Ag|PEO-Ag+, Ag|poly(ethylene glycol) monolayer-PEO-Ag+, and Ag|poly(ethylene glycol) monolayer-PEO-Ag+/Ag-nanoparticle. Statistical analysis shows that the distribution of formation times exhibits Gaussian behavior, and the fastest average initial formation time occurs for the Ag|PEO-Ag+ system. In contrast, formation in the presence of Ag nanoparticles likely proceeds by a noncontact bipolar electrochemical mechanism, exhibiting the slowest initial filament formation. Dissolution times are log-normal for all three systems, and repeated reformation of filaments from previously formed structures is characterized by rapid regrowth. The direct-write bipolar electrochemical deposition/dissolution strategy developed here presents an approach to reconfigurable, noncontact in situ wiring of nanoparticle arrays-thereby enabling applications where actively controlled connectivity of nanoparticle arrays is used to manipulate nanoelectronic and nanophotonic behavior. The system further allows for facile manipulation of experimental conditions while simultaneously characterizing surface conditions and filament formation/dissolution kinetics.

Formation/Dissolution of Conductive Silver Filaments through an Ionic Liquid/Polymer Electrolyte Thin Film

Materials with reconfigurable optical properties are potential candidates for applications such as optical cloaking and wearable sensors. One approach to develop these materials is to create and destroy atomic-scale conductive channels in well-defined locations within a polymer film via field-effect. By applying an external field to a solid polymer electrolyte, silver filaments can be created and destroyed on demand. However, this approach requires a balance between fast ion mobility for fast filament formation/dissolution, along with robust mechanical properties for a flexible film with high elastic modulus. Unfortunately, the properties that promote fast ion transport (e.g., polymer mobility in a solid polymer electrolyte) also degrade the mechanical properties. To address this challenge, a UV-crosslinkable polymer is combined with an ionic liquid and a silver salt. The crosslinkable polymer provides mechanical structure, while the ionic liquid serves as a non-volatile plasticizer to increase ion transport. Poly(ethylene glycol) diacrylate (PEGDA) is the crosslinkable polymer, 1-butyl-3-methylimidazolium tetrafluoroborate is the ionic liquid, and AgBF4 is the silver salt. Ionic liquids are chemically and thermally stable with negligible vapor pressure, high ionic conductivity, and a wide electrochemical window. 100 nm thick films of 80/20 and 60/40 wt% PEGDA/IL are spin-coated, annealed, and measured by AFM inside an argon-filled glovebox where O2 and H2O are controlled to < 0.1 ppm. Using a conducting AFM tip as one electrode and a silver substrate as the other, automated measurement of hundreds of filament formation/dissolution events are made at predefined locations in a single experimental session. Filament formation occurs at a 3 V forward bias over timescales that range five orders of magnitude (Fig. (a)), while dissolution occurs at a 3 V reverse bias on the timescale of hundreds of milliseconds. As shown in Fig. (a), the formation time distributions for both 60/40 and 80/20 PEGDA/IL samples are bimodal: the fast formation times occur within a few milliseconds while the slow can require several hundreds of seconds. This is likely a consequence of IL-rich and poor regions within the film, as supported by AFM modulus measurements showing regions of high and low elastic modulus (Fig. (b)). Filaments are expected to form faster in regions of low modulus, and slower in regions of high modulus. This explanation is further supported by comparing the samples containing 60 versus 80 wt% PEGDA: the distribution shifts towards increasing formation times with increasing polymer concentration. These results show that the formation/dissolution kinetics can be controlled by controlling the microstructure of the film, and a more homogeneous microstructure will be required to narrow the distribution of formation/dissolution times. Acknowledgement: This worked was supported by the Defense Advanced Research Projects Agency (DARPA) Atoms to Products (A2P) program, grant #FA8650-15-C-7546. Figure 1

Analysis of quantum conductance, read disturb and switching statistics in HfO2 RRAM using conductive AFM

Most studies on resistance switching have been carried out at the device level with the standard electrical characterization setup, which allows for effective automated reliability test and extensive characterization of the lifetime of an RRAM device. However, it is equally important to be able to probe the switching phenomenon at the nanoscale so as to improve insight on the bias-dependent kinetic behavior of the filament during multiple reversible breakdown and recovery cycles. This study aims to do just that by probing HfO2 blanket films (~4nm) with a W bottom electrode using an ultra-sharp Pt-wire conductive AFM (CAFM) tip with an areal resolution of ~10–20nm at ambient conditions. The use of the CAFM allows for a more reliable assessment of single filament evolution behavior as possible multiple filamentation events (common at the device level) are rare for such small probing areas. The role of oxygen vacancy induced filaments is studied here by using low compliance setting and moderate voltage levels, ensuring operation in the sub-quantum conductance regime. Our results show good repeatable switching trends and also provide insight on the quantum conductance phenomenon in oxygen vacancy based filaments. The read disturb trends in switching are investigated for the high resistance state (HRS) and the impact of tip-induced mechanical stresses on forming lifetime is also presented, which could serve as a motivator for further studies on non-volatile memory (NVM) reliability for flexible electronics devices and system on chip (SoC) applications.

Magnetoelectric oxide films for spin manipulation in graphene

The challenge of creating a graphene spin field effect transistor (spin-FET) demands a magnetic gate dielectric material whose magnetization can be switched electrically. We have grown films of Cr2O3 on top of graphite and graphene by pulsed laser deposition that shows this crucial functionality. We demonstrate that the Cr2O3 films are magnetoelectric by poling them in combined electric and magnetic fields and then using magnetic force microscopy to observe spontaneous surface domain structure as a function of poling field. In addition, we show that the electric field created by a conducting AFM tip can be used to write magnetic patterns in the film that demonstrate the kind of continuous magnetoelectric control needed for a prototype spin-FET. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

INITIATION OF POLARIZED STATE IN LITHIUM NIOBATE THIN FILMS SYNTHESIZED ON ISOLATED SILICON SUBSTRATES

In 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. 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.

Theoretical and practical approach to overcome curvature radius limitation of conductive atomic force microscopy tip for imaging of advanced technological node static random access memory devices

This paper demonstrates a methodology to overcome the challenges in obtaining a distinguishable current image when the radius of curvature of the conductive atomic force microscopy (C-AFM) tip is larger than the feature size of the device under test, e.g., the individual tungsten contacts of a static random access memory (SRAM) device. A model is presented to understand the interaction between the C-AFM tip and the features on the surface of a SRAM sample. Based on this model, recessed depth (tungsten recessed to neighboring dielectric) or protruding height (protruding tungsten with respect to the dielectric) is then created on the sample surface to provide suitable topographical contrast so that the electrical signals from the smaller tungsten contacts could be better discerned. It has been calculated that the surface treated dimensions need to fall within a feasible range in order to obtain a well-resolved current image. In the case where a recessed depth is created, the depth cannot be too deep as the edge of the neighboring insulating dielectric would block the base of the C-AFM tip and prevent the tip from accessing the lower-residing tungsten contact. The lack of good contact between the tip and receding tungsten contact would result in the inability to extract the electrical signals from the tungsten contact. In the case where protruding tungsten is created, a height limit exists whereby the conductive-AFM tip would unintentionally touch the neighboring tungsten contacts, creating a false illusion that the tungsten contacts are shorted together. The proposed model is generic and is able to serve as a guide to achieve a good current image. The required dimension could be easily calculated by inserting the known specifications into the model. Following the theoretical understanding, the recessed depth and protruding height are then physically created on the sample surface for enhanced C-AFM imaging. The experimental results agree well with the theoretical predictions from the model. This technique is then applied on actual failure analysis/fault isolation of SRAM devices.

Microscopic and macroscopic characterization of the charging effects in SiC/Si nanocrystals/SiC sandwiched structures

Microscopic charge injection into the SiC/Si nanocrystals/SiC sandwiched structures through a biased conductive AFM tip is subsequently characterized by both electrostatic force microscopy and Kelvin probe force microscopy (KPFM). The charge injection and retention characteristics are found to be affected by not only the band offset at the Si nanocrystals/SiC interface but also the doping type of the Si substrate. On the other hand, capacitance–voltage (C–V) measurements investigate the macroscopic charging effect of the sandwiched structures with a thicker SiC capping layer, where the charges are injected from the Si substrates. The calculated macroscopic charging density is 3–4 times that of the microscopic one, and the possible reason is the underestimation of the microscopic charging density caused by the averaging effect and detection delay in the KPFM measurements.

Template Assisted Growth of Zinc Oxide-Based Nanowires and Piezoelectric Properties

In this paper, we report on the synthesis of ZnO nanowires via templated electrochemical deposition. ZnO nanowire arrays were fabricated by potentiostatic electrodeposition in track-etched polycarbonate (PC) membrane. The electrolyte was aqueous solution containing zinc nitrate precursor. The electrodeposition process involves the electroreduction of nitrate ions to alter the local pH and precipitation of the metal oxide within the pores. The morphology analysis and structure characterization of the ZnO nanowires were carried out using conventional scanning electron microscopy (SEM) and X-Ray diffraction. To check the piezoelectric characteristics of the zinc oxide nanowires, the AFM microscope is used in contact mode. The scanned area was 5µm*5µm and the affected force was 30nN. In result of scanning each nanowire with conductive AFM tip in contact mode, a current peak which had a width smaller than topography peak was fabricated. This is due to semi-conductivity and piezoelectricity characteristics of Nanowires.

Fast Current Blinking in Individual PbS and CdSe Quantum Dots

Fast current intermittency of the tunneling current through single semiconductor quantum dots was observed through time-resolved intermittent contact conductive atomic force microscopy in the dark and under illumination at room temperature. The current through a single dot switches on and off at time scales ranging from microseconds to seconds with power-law distributions for both the on and off times. On states are attributed to the resonant tunneling of charges from the electrically conductive AFM tip to the quantum dot, followed by transfer to the substrate, whereas off states are attributed to a Coulomb blockade effect in the quantum dots that shifts the energy levels out of resonance conditions due to the presence of the trapped charge, while at the same bias. The observation of current intermittency due to Coulomb blockade effects has important implications for the understanding of carrier transport through arrays of quantum dots.

Piezoelectric properties and surface potential behavior in LiNbO3 thin films grown by the radio frequency magnetron sputtering

Abstract In this paper we report on using the atomic force microscope in piezoresponse and Kelvin modes to image grains, ferroelectric domains and surface potential in lithium niobate thin films. The radio frequency magnetron sputter system was used to deposit LiNbO 3 thin films on (1 0 0)-oriented Si substrate with SiO 2 layer. The surface of the sample showed small grains which diameter ranged from 70 nm to 130 nm and roughness less than 13 nm. Using the electric field from the biased conducting AFM tip showed that it was possible to form and subsequently visualize the ferroelectric state in LiNbO 3 thin films synthesized on electrically isolated silicon substrate. We also report on surface charge retention on ferroelectric thin films by Kelvin probe microscope in comparison with the piezoresponse signal. We also regard the possibility of retention of surface charge obtained by Kelvin probe microscopy in comparison with piezoresponse signal on ferroelectric thin films.

Nanoscale quantification of charge injection and transportation process in Si-nanocrystal based sandwiched structure

Si nanocrystals are formed by using KrF pulsed laser crystallization of an amorphous SiC/ultrathin amorphous Si/amorphous SiC sandwiched structure. Electrons and holes are injected into Si nanocrystals via a biased conductive AFM tip and the carrier decay and transportation behaviours at the nanoscale are studied by joint characterization techniques of Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM). Quantification of the surface charge density is realized by solving the Poisson equation based on KPFM measurements. Besides, the asymmetric barrier height for electrons and holes is considered to play a dominant role in controlling the charge retention and transportation characteristics. The methodology developed in this work is promising for studying the charge injection and transportation process in other materials and structures at the nanoscale.