Monitoring Fast Thermal Dynamics at the Nanoscale through Frequency Domain Photoinduced Force Microscopy

Abstract

In illuminated tip–sample junctions, the absorption of light by the sample is accompanied by local heating and subsequent thermal expansion of the material. In photoinduced force microscopy (PiFM) experiments, thermal expansion is expected to affect the measured photoinduced force through the thermally modulated van der Waals force. Evidence for such thermal contributions in PiFM measurements has been demonstrated in the mid-infrared range, where the primary excitations are molecular vibrational modes. For PiFM measurements in the vis/NIR, where light-matter energy transfer is mediated through electronic excitations, clear experimental evidence of thermal contributions remains elusive. By developing a frequency domain version of PiFM, we retrieve variations in the photoinduced force on the sub-μs time-scales, allowing a direct registration of the thermal relaxation dynamics of the sample after photoexcitation. Our measurements confirm the presence of the thermal contribution to the PiFM signal in the mid-infrared range and provide strong experimental evidence that thermal components also play a role in the forces measured in PiFM in the vis/NIR range of the spectrum.

Nanosecond-resolution photothermal dynamic imaging via MHZ digitization and match filtering

“…Our results show that careful choice of detection method has to be made for optimal sensing of the photothermal signal induced by a short pulse excitation. While the same level of SNR improvement may be achieved by using other advanced demodulation techniques that are capable of capturing those high-frequency photothermal signals, such as signal synthesis using lock-in multi-channel demodulation 48 , 49 or boxcar averaging 50 , those advanced demodulation methods are not yet widely available. In contrast, the broadband detector and digitizer or MHz are readily available and highly cost-effective.…”

Surface Modification of ETFE Membrane and PTFE Membrane by Atmospheric DBD Plasma

“…To verify our hypothesis, IR PiFM was applied to analyze the surface chemistry of ETFE-0 and ETFE-3 membranes at the nano-layer level. The IR PiFM technique has been reported to resolve vibrational signals between a probing tip and material surface [ 20 , 21 , 22 ], obtaining infrared spectroscopy results with a resolution up to the nanometer level [ 23 ]. Taking ETFE-0 as an example, the nano-level infrared spectra of the membrane surface obtained by IR PiFM ( Figure 4 a) showed that a C–H deformation vibration peak at 1453 cm −1 and C–F peak at 950–1400 cm −1 , which are consistent with the results obtained in ATR-FTIR ( Figure 2 a).…”

Photo induced force microscopy: chemical spectroscopy beyond the diffraction limit

“…Jahng et al 's model 33 considered incident light causing a thermal expansion at the tip/sample junction and generating a force interaction through two processes: a “direct” contribution resulting from far-field transmission and absorption by the sample and an “indirect” contribution caused by the enhanced near field, generated by the antenna effect of the tip. The model indicated that the thermally stimulated forces would be as much as a hundred times stronger than the induced dipole force for most organic materials for mid infrared radiation and recent experimental data, utilising an innovative frequency domain force detection method that allows sub-μs time measurements, 34 suggests the same is true in the visible/near infrared region where energy transfer is through electronic excitations.…”

“…27,28 The resulting gradient force, F g , depends upon the polarisation of the particle in the z direction and because of its 1 Z 4 dependence, is highly localised that can be measured with very high spatial resolution. [29][30][31] Subsequently, experimental evidence accumulated that the cantilever response was dominated by dissipative effects rather than the dispersive response predicted by the dipole-dipole force model, 33,34 thus indicating the involvement of a thermal process in generating PiFM contrast. Jahng et al's model 33 considered incident light causing a thermal expansion at the tip/sample junction and generating a force interaction through two processes: a ''direct'' contribution resulting from far-field transmission and absorption by the sample and an ''indirect'' contribution caused by the enhanced near field, generated by the antenna effect of the tip.…”