The demand for smaller, faster, and more efficient electronic architectures and devices has produced an urgent need to understand and mitigate the associated deleterious thermal effects. As a result, significant efforts have been devoted to the development of experimental methods that quantify nanoscale thermal transport with both high spatial and temporal resolutions, bringing about new insights into a variety of phenomena; including thermal conductivity, thermal strain, and phonon behaviors . This illustrates the value of extending time-averaged metrology techniques into the combined ultrafast and nanoscale regimes, which are the native scales on which thermal-energy carriers operate. In light of this, we are exploring ultrafast electron microscopy (UEM) as a method to access this challenging parameter space. In a typical UEM experiment, a femtosecond (fs) laser pulse is used to excite (pump) the specimen in situ, upon which a fs-duration discrete packet of photoelectrons generated from the TEM electron source is used to probe the response . By varying the relative arrival times of the pump laser pulses and the probe electron packets at the specimen, nanometer-scale ultrafast structural responses can be directly imaged, while angstrom-scale responses can be tracked in reciprocal space. In UEM, as in other ultrafast electron and X-ray experiments, the Debye-Waller (DW) effect is often invoked when explaining photoinduced scattering intensity changes and when determining transient temperatures by relating the changes to atomic thermal vibrations. While this enables direct probing of the intrinsic lattice response to thermal effects with high spatial and temporal resolutions, reliance on transient intensity changes of diffracted beams poses significant challenges. This is because factors other than mean-square atomic displacements can produce signal changes similar to those generated by thermal effects [3-5]. Therefore, in order to accurately and precisely determine nanoscale transient temperatures using ultrafast scattering methods, deleterious effects that obfuscate intensity changes arising from atomic thermal vibrations must be identified, quantified, and deconvolved.
To address this, we have systematically studied and quantified a number of potential effects that can arise during in situ fs photoexcitation that may impact DW-type responses and the resulting interpretations . Using small-grained polycrystalline aluminum films as a well-characterized test system, we explicitly illustrate the impact of specimen tilting and translation via rigorous statistical analysis of Debye-Scherrer-ring intensities obtained from numerous individual specimens and measurements. Despite having more than 105 individual grains within the selected areas, we find that tilting by as little as a fraction of a degree, or translating a 22.5-μm diameter illuminated region by only 20 nm, yields statistically-significant changes in the diffracted-beam intensity. This result is at first surprising, as one may have expected a uniform distribution of random zone-axis orientations under such conditions, thus circumventing any negative effects due to specimen tilting and translation. We explicitly show this is not the case, and we explore and quantify several sources of error and artifacts, such as slight texturing of the polycrystalline films due to surface-energy minimization. Possible approaches to mitigating inaccuracies in the DW-calculated temperatures are also addressed, including effective methods of data normalization and consideration of ideal specimen geometries for such studies.
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