Nanoscale Mapping of Dielectric Properties of Nanomaterials from Kilohertz to Megahertz Using Ultrasmall Cantilevers Maria J. Cadena,†,‡ Seung Hyun Sung,¶ Bryan W. Boudouris,¶ Ronald Reifenberger,§,‡ and Arvind Raman*,†,‡ †
School of Mechanical Engineering, Purdue University, West Lafayette, Indiana 47907, United States Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, United States ¶ School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States § Department of Physics, Purdue University, West Lafayette, Indiana 47907, United States ‡
ABSTRACT: Electrostatic force microscopy (EFM) is often used for nanoscale dielectric spectroscopy, the measurement of local dielectric properties of materials as a function of frequency. However, the frequency range of atomic force microscopy (AFM)-based dielectric spectroscopy has been limited to a few kilohertz by the resonance frequency and noise of soft microcantilevers used for this purpose. Here, we boost the frequency range of local dielectric spectroscopy by 3 orders of magnitude from a few kilohertz to a few megahertz by developing a technique that exploits the high resonance frequency and low thermal noise of ultrasmall cantilevers (USCs). We map the frequency response of the real and imaginary components of the capacitance gradient (∂C(ω)/∂z) by using second-harmonic EFM and a theoretical model, which relates cantilever dynamics to the complex dielectric constant. We demonstrate the method by mapping the nanoscale dielectric spectrum of polymer-based materials for organic electronic devices. Beyond offering a powerful extension to AFM-based dielectric spectroscopy, the approach also allows the identification of electrostatic excitation frequencies which affords high dielectric contrast on nanomaterials. KEYWORDS: ultrasmall cantilevers, EFM, dielectric spectroscopy, second harmonic, capacitance gradient
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However, traditional BDS techniques are unable to perform spatially sensitive measurements, which probe local modifications introduced by nanostructures, boundaries, and interfaces on dielectric properties. Local probes are particularly important in understanding the dielectric properties of composite films. Lacking spatially resolved measurements, the interpretation of experimental results from macroscopic samples often becomes a model-dependent exercise.9−11 To enable spatially sensitive dielectric spectroscopy measurements at the mesoscopic length scale, a number of atomic force microscopy (AFM) methods have been developed recently. For example, the glassy dynamics of polyvinyl acetate (PVAc) films were studied at one spatial point as a function of temperature at low frequencies (0.01 < f < 100 Hz) using frequency modulation electrostatic force microscopy (FM-EFM).12,13 In a similar study, the dielectric relaxation of a polystyrene/PVAc blend was analyzed at a single point, using phase images at one
easurements of dielectric properties are relevant to the understanding of electrical and structural characteristics of heterogeneous materials in a wide range of devices, such as organic emitting diodes, photovoltaic cells and nonvolatile memories.1−3 In such applications, novel materials comprising different nanoscale domains (phases) are continuously emerging. Controlling the dielectric performance of the different components and the mixture as a whole presents a formidable challenge.4−6 The dielectric response of a material under the influence of an applied field is driven by three main frequency-dependent polarization mechanisms: intra-atomic (electronic), interatomic (vibrational), and orientational.7 Frequency dependence of bulk materials is often determined using broadband dielectric spectroscopy (BDS). This is a popular technique for bulk measurements of dielectric properties over a wide range of frequencies, ranging from 90°), keeping constant the distance between the tip and the sample. Lock-in B generates the electrical excitation signal at a frequency ωe and amplitude VAC. Lock-in C is set to measure the electrostatic interaction force at 2ωe through the amplitude and phase observables, A2ωe and ψ2ωe, respectively. It is used to track the spatial dependence of ∂C/∂z. A cypher AFM is used to acquire the data. An important feature of this instrument that enables the use of USCs is the spot size of the laser, which is about 3 μm. All measurements were made under ambient conditions. The main observables from the experiments are topography and amplitude (A2ωe) and phase (ψ2ωe) from the cantilever response at the second harmonic of the electrostatic excitation. For a complete data set over a broad frequency range, a set of experiments was systematically performed on each sample, acquiring the cantilever response due to electrostatic excitation at 24 equally spaced logarithmic values of frequency ranging from 8 kHz to 2 MHz. The maps taken at each frequency were used to calculate the real and imaginary capacitance gradient as a function of frequency using eqs 9 and 10. Metalized USC were used under ambient conditions to obtain each set of data. A gold coating is on both sides of the USC, but the tip itself is left uncoated. Therefore, sputtering is used to deposit a thin layer of titanium (5 nm) followed by gold (20 nm). Table 3 specifies the relevant parameters for the USC and the ANSCMA-PA probe used for comparison in Figure 1. To estimate Sqq(ω) in Figure 1b, we measured the cantilever deflection q(t) in thermal motion from the photodetector at a sampling rate of 80 MS/s. Then, segment averaging and windowing using a Hann function with 50% overlap is applied to the collected
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Table 3. Relevant Properties of a Conventional Long Cantilever (ANSCM-PA from App-Nano) and a Ultrasmall (USC-F5-k30 from NanoWorld) Cantilever parameters
ANSCM-PA
USC
ω0 k Q length width tip height
240 kHz 25.8 N/m 449.1 125 μm 30 μm 14 μm
4.8 MHz 27.4 N/m 381.1 10 μm 5.4 μm 2.5 μm 4069
DOI: 10.1021/acsnano.5b06893 ACS Nano 2016, 10, 4062−4071
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