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Title:Nanoscale infrared absorption imaging and tomography
Author(s):Kenkel, Seth
Director of Research:Bhargava, Rohit
Doctoral Committee Chair(s):Sinha, Sanjiv
Doctoral Committee Member(s):Cahill, David; Aluru, Narayana
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Abstract:Near-field spectroscopic imaging techniques offer a non-perturbative, molecular contrast for characterization of nanomaterials. While several techniques now provide nanoscale chemical maps by combining molecular vibrational spectroscopies with Atomic Force Microscopy (AFM), data are often complicated by the measurement apparatus, light matter interaction scheme and the low signal to noise ratio. Together, these effects have not permitted reliable analytical measurements in a near-field configuration. In this thesis, we study the contrast mechanism of contact mode Atomic Force Microscopy – Infrared (AFM-IR) spectroscopic imaging to enable accurate, nanoscale infrared absorption imaging. We first describe the analytical solution of a rectangular cross-section cantilever beam mechanically coupled to a vertically-perturbed surface. From this understanding, we show the AFM-IR signal (recorded using heterodyne detection) can be described as the product of the local photo-induced, sample expansion and the cantilever resonance response (responsivity). We use the insight obtained to develop a new approach for removing cantilever responsivity contrast by recording an additional signal resulting from a constant reference expansion generated by a mechanical actuator (piezo) placed under the sample. Dividing the raw AFM-IR contrast by this reference signal at each sample position eliminates spatial responsivity variations enabling accurate chemical imaging. Although the ratio approach corrects for spatial responsivity variations, time-dependent changes in responsivity produce noise which scales with cantilever deflection. Next, we demonstrate a closed-loop controller for the piezo actuator to maintain zero deflection through destructive interference of the piezo and photo-induced expansions. Measuring the piezo voltage offers an alternative AFM-IR measurement with low susceptibility to both spatial and temporal changes in responsivity improving the SNR by >5x. Last, we describe an analytical Green’s function derived from Thermoelasticity relating 3 dimensional heat generation to vertical surface expansion for a n-layer laminated, half-space. From this theory, we show the image resolution is dependent on parameters such as sample thickness and material properties and propose a new method for distinguishing absorption in 3 dimensions harnessing the frequency dependent thermal diffusion.
Issue Date:2020-05-07
Rights Information:Copyright 2020 Seth Kenkel
Date Available in IDEALS:2020-08-27
Date Deposited:2020-05

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