Intermodulation electrostatic force microscopy for imaging surface photo-voltage

Abstract

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...

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“…Surface potential imaging is acquired using intermodulation electrostatic force microscopy (ImEFM, Intermodulation Products) [26]. ImEFM is an alternative to Kelvin probe force microscopy, which uses resonance to enhance the force sensitivity of the cantilever (MikroMasch HQ:NSC15/Pt), allowing for a much improved signal-tonoise ratio in a given measurement bandwidth (inverse of the signal integration time at each pixel).…”

Recent advances in hybrid measurement methods based on atomic force microscopy and surface sensitive measurement techniques

“…342 Ongoing research is devoted to enhancing the time resolution, because the scan speed of FM mode is slow 343 and AM mode measurements are burdened with artifacts, such as the stray capacitance effect 344 and interfering signals, leading to topographical coupling. 345,346 Several approaches try to couple the lateral resolution and reproducibility of FM mode KPFM with enhanced time resolution similar to AM mode KPFM, such as time-resolved electrostatic force microscopy, pump-probe KPFM, 347 general acquisition mode, 348 open loop (OL) KPFM 349 and Heterodyne (H) KPFM. 350 For example, H-KPFM improved the scan rate while maintaining the spatial resolution and voltage sensitivity comparable to FM-KPFM.…”

Особенности Измерения Поверхностного Распределения Электрического Потенциала Локальным Зондом На Основе Полевого Транзистора С Каналом-Нанопроводом

“…Для проведения многих исследований в микроэлектронике, биологии, химии и медицине необходим высокочувствительный неразрушающий анализ пространственного распределения профиля электрического потенциала с высоким пространственным и полевым разрешением. К настоящему времени известны десятки методик сканирующей зондовой микроскопии (СЗМ) для измерения электрических свойств твердотельных или мягких поверхностей [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. Электростатическая силовая микроскопия (EFM) и силовая микроскопия зонда Кельвина (KFM) основаны на эффекте электростатического взаимодействия между смещенным зондом и образцом для расчета количественного значения электрического потенциала.…”