A J Lockwood

Friction-formed liquid droplets.

The formation of nanoscale liquid droplets by friction of a solid is observed in real-time. This is achieved using a newly developed in situ transmission electron microscope (TEM) triboprobe capable of applying multiple reciprocating wear cycles to a nanoscale surface. Dynamical imaging of the nanoscale cyclic rubbing of a focused-ion-beam (FIB) processed Al alloy by diamond shows that the generation of nanoscale wear particles is followed by a phase separation to form liquid Ga nanodroplets and liquid bridges. The transformation of a two-body system to a four-body solid-liquid system within the reciprocating wear track significantly alters the local dynamical friction and wear processes. Moving liquid bridges are observed in situ to play a key role at the sliding nanocontact, interacting strongly with the highly mobile nanoparticle debris. In situ imaging demonstrates that both static and moving liquid droplets exhibit asymmetric menisci due to nanoscale surface roughness. Nanodroplet kinetics are furthermore dependent on local frictional temperature, with solid-like surface nanofilaments forming on cooling. TEM nanotribology opens up new avenues for the real-time quantification of cyclic friction, wear and dynamic solid-liquid nanomechanics, which will have widespread applications in many areas of nanoscience and nanotechnology.

The formation of carbon nanostructures by in situ TEM mechanical nanoscale fatigue and fracture of carbon thin films

A technique to quantify in real time the microstructural changes occurring during mechanical nanoscale fatigue of ultrathin surface coatings has been developed. Cyclic nanoscale loading, with amplitudes less than 100 nm, is achieved with a mechanical probe miniaturized to fit inside a transmission electron microscope (TEM). The TEM tribological probe can be used for nanofriction and nanofatigue testing, with 3D control of the loading direction and simultaneous TEM imaging of the nano-objects. It is demonstrated that fracture of 10-20 nm thick amorphous carbon films on sharp gold asperities, by a single nanoscale shear impact, results in the formation of <10 nm diameter amorphous carbon filaments. Failure of the same carbon films after cyclic nanofatigue, however, results in the formation of carbon nanostructures with a significant degree of graphitic ordering, including a carbon onion.

Advanced transmission electron microscope triboprobe with automated closed-loop nanopositioning

Here the design and operation of a novel transmission electron microscope (TEM) triboprobe instrument with real-time vision control for advanced in situ electron microscopy is demonstrated. The NanoLAB triboprobe incorporates a new high stiffness coarse slider design for increased stability and positioning performance. This is linked with an advanced software control system which introduces both new and flexible in situ experimental functional testing modes, plus an automated vision control feedback system. This advancement in instrumentation design unlocks new possibilities of performing a range of new dynamical nanoscale materials tests, including novel friction and fatigue experiments inside the electron microscope.

Characterising performance of TEM compatible nanomanipulation slip-stick inertial sliders against gravity

Slick-stick inertial sliders have been used in the manufacture of a novel miniaturised nanomanipulation and nanoindentation system. This has been designed to perform sub-micron localised in-situ deformation studies in a high resolution transmission electron microscope (HRTEM). Coarse position is realised by three independent sliders which are set mutually perpendicular to one another (x, y and z) with a range > lmm and resolution <100nm In this paper we discuss the effect that gravity has on the performance of the nanomanipulation slip-stick inertial drives by monitoring displacement rate, resolution and the effect of the normal force across the slip-stick slider. We report on the successful actuation of slip-stick sliders operated in a vertical orientation.

Cyclic deformation and nano-contact adhesion of MEMS nano-bridges by in-situ TEM nanomechanical testing

MEMS nano-bridges fabricated by FIB have been deformed in-situ in the TEM. The polysilicon bridges show high levels of flexibility but also, at increased indentation depths, residual plastic deformation after fully unloading the bridges. Here, a significant number of cycles were applied to the centre of a bridge by a W-probe. This resulted in the formation of an adhesive contact with the W-probe. On unloading the nano-bridge regained its original shape and then deformed upwards, adhered to the W-probe. A significant high tensile force of -17μN was required to sever the nano-contact. Analysis of the W-probe and polysilicon nano-bridge indicate that carbon migration along the W-probe and local contact heating due to the associated fatigue cycles were responsible for the adhesive bond, with initial carbon contamination layer on the W-probe of 2nm, thickening during the loading cycles to 25nm.

Characterising ambient and vacuum performance of a miniaturised TEM nanoindenter for in-situ material deformation

A miniaturised nanomanipulation and nanoindentation system has been designed and manufactured to perform sub-micron localised in-situ deformation studies in a high resolution transmission electron microscope (HRTEM). The nanomanipulation drive comprises two independent mechanisms for both coarse and fine positioning of sharp indenter tips. Small slip-stick inertial sliders are used to coarsely position a tip which fits inside a bespoke hollowed specimen holder for a JEOL (Japan) 2010/3010 series microscope. The coarse drive comprises three fully independent sliders which are set mutually perpendicular to one another (x, y and z) with a range > 1 mm and resolution ~ 100 nm. Fine positioning is achieved with a quartered piezoelectric tube with range ~ 2 μm and resolution < 1 A. Optical displacement sensors have been used to characterise the nanomanipulation drive performance including total displacement rate and step size in ambient conditions. These are compared to the operation of the drive within a TEM under vacuum conditions. TEM observations at high magnification enable optimisation of the fine and coarse motion and overall drive stiffness.

Deformation behaviour of polysilicon components for MEMS

The mechanical performance of miniaturized Si components depends on both microstructure and geometry. Nanoscale polysilicon beams with both fixed–fixed and portal–frame-type beam geometries have been experimentally deformed and modelled using FEA. It is shown here that the combination of both geometry of a beam support and nanoscale dimensions results in a significantly different mechanical response of beams to that predicted using bulk properties. In a fixed–fixed configuration, beams exhibit limited elasticity followed by rapid failure occurring at the beam mid-point. However, by allowing the beam to have a less rigid connection to the substrate as in the case for a portal–frame geometry, a beam of similar dimensions can undergo significantly more deformation (∼five times more), without failure. The increased flexibility of the support connection results in lower stresses particularly at critical positions along the beam and a lower maximum tensile stress of ∼35–40% less than for a fixed–fixed beam geometry. We also show that for cyclic deformation, a build-up in residual beam deformation is related to changes in microstructure, which can relax back to a less deformed state without continuously applied high deformation levels.

A piezoelectric goniometer inside a transmission electron microscope goniometer.

Piezoelectric nanoactuators, which can provide extremely stable and reproducible positioning, are rapidly becoming the dominant means for position control in transmission electron microscopy. Here we present a second-generation miniature goniometric nanomanipulation system, which is fully piezo-actuated with ultrafine step size for translation and rotation, programmable, and can be fitted inside a hollowed standard specimen holder for a transmission electron microscope (TEM). The movement range of this miniaturized drive is composed of seven degrees of freedom: three fine translational movements (X, Y, and Z axes), three coarse translational movements along all three axes, and one rotational movement around the X-axis with an integrated angular sensor providing absolute rotation feedback. The new piezoelectric system independently operates as a goniometer inside the TEM goniometer. In situ experiments, such as tomographic tilt without missing wedge and differential tilt between two specimens, are demonstrated.

A novel tripod-driven platform for in-situ positioning of samples and electrical probes in a TEM

We present a design for a novel coarse positioning system based on a tilting platform which is positioned using linear slip/stick motors. The design differs from common arrangements of stacked x, y, and z motors, and also ball mounted slip/stick motors, by allowing easy access along the central axis of the microscope holder. The drive motors are highly compact and co-linear and may be easily incorporated in an off-axis configuration, leaving a central cylindrical region with an approximate diameter of 3mm which is available to accommodate screened electrical wiring and optical fibres. We show that the tripod can be used to manoeuvre two metallic tips towards each other in-situ in a TEM in nanometre-scale lateral steps.

In-situ TEM mechanical testing of a Si MEMS nanobridge

The failure mechanisms of micro-electromechanical systems (MEMS) are an area of current interest as many aspects of MEMS failure are somewhat uncertain despite MEMS usage and applications dramatically increasing. Current understanding of Si MEMS failure is generally based on oxide thickening and stress corrosion cracking (SCC) [1], leading to catastrophic failure. Testing and observing MEMS structures within a vacuum is becoming more widespread, for example actuating the MEMS devices over many thousands of cycles inside scanning electron microscopes (SEM) [2]. This SEM technique is useful in understanding cycles to failure, however does not address the effects that microstructure has upon failure. One method to understand how failure occurs with respect to the microstructure is to analyse the failure post mortem using a transmission electron microscope (TEM). This technique does not address dynamic microstructural effects during operation and failure. Recent advancements in TEM are exploring dynamic testing using various in-situ techniques such as atomic force microscopy (AFM)-TEM, scanning probe microscopy (SPM)-TEM and TEM-nanoindentation. Real-time observations made during mechanical probing of nanosized structures are currently being undertaken using in-situ TEM nanoindentation.