Atomic force microscope (AFM) users often calibrate the spring constants of cantilevers using functionality built into individual instruments. This is performed without reference to a global standard, which hinders robust comparison of force measurements reported by different laboratories. In this article, we describe a virtual instrument (an internet-based initiative) whereby users from all laboratories can instantly and quantitatively compare their calibration measurements to those of others -standardising AFM force measurements -and simultaneously enabling noninvasive calibration of AFM cantilevers of any geometry. This global calibration initiative requires no additional instrumentation or data processing on the part of the user. It utilises a single website where users upload currently available data. A proof-of-principle demonstration of this initiative is presented using measured data from five independent laboratories across three countries, which also allows for an assessment of current calibration.

We use a recently developed scanning probe technique to image with high spatial resolution the injection and extraction of charge around individual surface-modified aluminum oxide nanoparticles embedded in a low-density polyethylene (LDPE) matrix. We find that the experimental results are consistent with a simple band structure model where localized electronic states are available in the band gap (trap states) in the vicinity of the nanoparticles. This work offers experimental support to a previously proposed mechanism for enhanced insulating properties of nanocomposite LDPE and provides a powerful experimental tool to further investigate such properties.

We demonstrate an alternative to Kelvin Probe Force Microscopy (KPFM) for imaging surface potential. The open-loop, single-pass technique applies a low-frequency AC voltage to the atomic force microscopy tip while driving the cantilever near its resonance frequency. Frequency mixing due to the nonlinear capacitance gives intermodulation products of the two drive frequencies near the cantilever resonance, where they are measured with high signal to noise ratio. Analysis of this intermodulation response allows for quantitative reconstruction of the contact potential difference. We derive the theory of the method, validate it with numerical simulation and a control experiment, and we demonstrate its utility for fast imaging of the surface photo-voltage on an organic photo-voltaic material.One of the most popular and useful methods of Electrostatic Force Microscopy (EFM) is Kelvin Probe Force Microscopy (KPFM)1 which provides a measurement of the contact potential difference V CPD (sometimes referred to as the surface potential). KPFM is widely used for advanced imaging of composite polymeric materials 2 and for imaging of the local work function on the surface of organic photo-voltaic materials 3 . Although KPFM is a useful technique to investigate electric properties of surfaces at the nanoscale, the signal-to-noise ratio, accuracy and speed are limited by the additional feed-back loops commonly used in its implementations 4 . To overcome these limitations, an open-loop technique was first proposed by Takeuchi et al.5 to image the contact potential difference in vacuum. Later the technique was used to measure the potential of nanoparticles in liquid 6 and to characterise ferroelectric thin films 7 . In this paper we propose and demonstrate an openloop technique that exploits the intermodulation (frequency mixing) of an electrostatic drive force and a mechanical drive force, to up-convert the electrostatic frequency to the first flexural resonance where the high quality factor allows for a more sensitive measurement. The contact potential difference can be imaged in a singlepass, allowing for imaging times shorter than 5 min with 256 × 256 pixel resolution.The electrostatic energy stored in a system of two perfect conductors is E EL = 1 2 CV 2 , where C is the capacitance and V the electrostatic potential difference between the two. The attractive electrostatic force is therefore;where z is the distance between the two conductors. In EFM the two conductors are the conductive tip and the a) Electronic mail: borgani@kth.se b) Electronic mail: haviland@kth.se sample substrate, which can be approximated as an axially symmetric electrode and an infinite conducting plane respectively. The resulting capacitance gradient varies as a non-linear function of z that depends on the tip geometry 8 . Intermodulation EFM (ImEFM) excites the cantilever with a shaker piezo at frequency ω D close to resonance ω 0 , while at the same time an AC voltage is applied to the cantilever at frequency ω E ≪ ω D . The total potential between t...

The interface between two wide band-gap insulators, LaAlO 3 and SrTiO 3 (LAO/STO) offers a unique playground to study the interplay and competitions between different ordering phenomena in a strongly correlated two-dimensional electron gas. Recent studies of the LAO/STO interface reveal the inhomogeneous nature of the 2DEG that strongly influences electrical-transport properties. Nanowires needed in future applications may be adversely affected, and our aim is, thus, to produce a more homogeneous electron gas. In this work, we demonstrate that nanostructures fabricated in the quasi-2DEG at the LaAlO 3 =SrTiO 3 interface, capped with a SrCuO 2 layer, retain their electrical resistivity and mobility independent of the structure size, ranging from 100 nm to 30 μm. This is in contrast to noncapped LAO/ STO structures, where the room-temperature electrical resistivity significantly increases when the structure size becomes smaller than 1 μm. High-resolution intermodulation electrostatic force microscopy reveals an inhomogeneous surface potential with "puddles" of a characteristic size of 130 nm in the noncapped samples and a more uniform surface potential with a larger characteristic size of the puddles in the capped samples. In addition, capped structures show superconductivity below 200 mK and nonlinear currentvoltage characteristics with a clear critical current observed up to 700 mK. Our findings shed light on the complicated nature of the 2DEG at the LAO/STO interface and may also be used for the design of electronic devices.

We describe a phase-coherent multifrequency lock-in measurement technique that uses the inverse Fourier transform to reconstruct the nonlinear current-voltage characteristic (IVC) of a nanoscale junction. The method provides for a separation of the galvanic and displacement currents in the junction, and easy cancellation of the parasitic displacement current from the measurement leads. These two features allow us to overcome traditional limitations imposed by the low conductance of the junction and high capacitance of the leads, thus providing an increase in measurement speed of several orders of magnitude. We demonstrate the method in the context of conductive atomic force microscopy, acquiring IVCs at every pixel while scanning at standard imaging speed.The sensitive measurement of small currents in nanometer-scale junctions is a central problem in modern experimental physics. Characterization of numerous novel materials and devices, in applications ranging from topological quantum computers [1, 2] to energy harvesting and energy conversion [3][4][5][6][7][8][9][10], struggles with the same basic limitations imposed by the small measurement current and the large stray capacitance of the macroscopic leads. We describe how to circumvent these limitations using phase-coherent multifrequency lock-in measurement and inverse Fourier transform to achieve a dramatic improvement in the speed of measurement, or alternatively, in the signal-to-noise ratio at the same measurement speed. In addition, our frequency-domain approach allows for active cancellation of parasitic current due to the lead capacitance and it provides for unambiguous separation of the galvanic and displacement currents flowing in the nanoscale junction.One area where this improvement is particularly useful is scanning probe microscopy (SPM), where a measurement of the nonlinear current-voltage characteristic (IVC) is desired at each tip location. In scanning tunneling microscopy (STM) the IVC allows for mapping energy dependence of the local density of electronic states [11]. In conducting atomic force microscopy (AFM) it can be used to map energy-conversion efficiency in photoactive nanocomposite materials [12]. Due to the aforementioned limitations, present-day SPM has two basic modes of operation. In imaging mode the current is measured while scanning the surface with a constant tip-sample voltage, quickly generating an image but with only limited information. Multiple scans at different bias are required to get the full IVC, greatly increasing measurement time and introducing problems due to instrument drift and tip wear. In spectroscopic mode the IVC is recorded at each tip position, but the voltage must be swept slowly so as to minimize displacement current in the parallel capacitance of the measurement leads. This large background current puts a limit on the achievable gain and sensitivity of current measurement, and the slow sweep greatly limits the speed of the scan, or equivalently spatial resolution in a given measurement time. We demonstra...