In a pre-proof article posted in Ultramicroscopy, researchers presented combined photothermal and photovoltaic effects produced by light absorption in the visible spectrum by atomic force microscopy (AFM) probes.
A thorough analysis of the modifications induced by the photothermal and photovoltaic effects of the atomic microscopy probe oscillation recorded in micromechanical oscillators was conducted. The findings demonstrated that the oscillation of the phase-voltage parabolic signals recorded in micromechanical oscillators exhibited distinct fingerprints. These fingerprints were based on light intensity and the characteristics of the materials in the atomic force microscopy probes.
The absorption of visible light can promote the excitation of electrons into higher energy levels in solid-state materials and is a physical phenomenon typically studied under optical absorption spectroscopy. Detecting forces induced by photothermal and photovoltaic effects by employing atomic force microscopy under wavelength-dependent light irradiation is a promising method for enhancing the spatial resolution beyond the diffraction limit of optical absorption spectroscopy.
AFM and its Applications in Optoelectronics
Using atomic force microscopy to cast light from external sources allows the examination of a wide range of optoelectronic phenomena with high spatial resolution. Precisely positioning the microscope tip at various elevations above the sample surface enables the analysis of optical-field improvement and other phenomena produced by the localization of light.
For many investigations where light-matter interactions at the sub-diffraction limit are active, the correlated optics technique and near-field force mapping is advantageous.
Most investigations have concentrated on studying the events induced by photothermal and photovoltaic effects in the material using atomic force microscopy-based techniques to advance spectroscopy and optical imaging beyond the diffraction limit. When exposed to incident light, the microscope force sensor affects the imaging and spectroscopic signals and complicates the detection of tip-sample interaction forces. Therefore, various excitation schemes have been designed to make recording the force signals induced by the photothermal and photovoltaic effects easier.
The current work demonstrated combined photothermal and photovoltaic effects caused by unmodulated visible light in Si- and Au-coated atomic force microscopy probes. The findings reported that the photothermal and photovoltaic effects on the atomic force microscopy probe independently shifted the voltage/phase parabola on the x- and y-axes, respectively. Additionally, the photothermal and photovoltaic effects on the atomic force microscopy probe were detected by evaluating the oscillation phase of the cantilever in the micromechanical oscillators as the external bias voltage function.
Notably, it was discovered that the photothermal effect raised the cantilever's temperature, shifting the phase signal. In contrast, the photovoltaic effect altered the total distributed charge of the tip apex (for Si tips), changing the sample contact potential difference (CPD). As the incident laser power changed, the photothermal effects shifted the phase linearly, while the photovoltaic effects changed the CPD values logarithmically.
Experimental Setup to Study the Combined Photothermal and Photovoltaic Effects
In this work, a Bruker atomic force microscope operated in a vacuum (p < 10-3 Torr) was used for the experiments. An external laser beam was directed on the tip through a coated glass slide. The supercontinuum Fianium SC400 laser beam achieved the optical excitation with the monochromator's slit set at 550 ± 5 nm.
The laser beam was focused within 8 mm on the sample. A high-frequency lock-in amplifier was used to monitor the phase signal by analyzing the cantilever oscillation phase in micromechanical oscillators with the piezoelectric actuation phase. The piezoelectric excitation's free oscillation amplitude in the micromechanical oscillators was tuned at roughly 20 nm. Additionally, two different types of antimony-doped Si probes were employed.
Atomic force microscopy was employed in a domain where the tip was far from the surface, thereby reducing the usual "surface forces" that would otherwise make it possible to visualize the surface topography. Long-range electrostatic forces, as opposed to short-range forces like Van de Waals, were the dominant forces that influenced cantilever vibration.
The parabola changed in two ways due to the incident laser's effects on the cantilever oscillation dynamics in the micromechanical oscillators. The photothermal effect first manifested as a parabolic shifting towards the negative phase values. Additionally, the parabola showed a displacement on the voltage axis, indicating the photovoltaic effect, which was observed as an alteration of the CPD value.
Damped harmonic micromechanical oscillators were used to characterize the oscillation dynamics of a cantilever in the micromechanical oscillators. Due to the lack of light modulation and the influence of photothermal effects enhanced by backside coatings, radiation pressure was not anticipated to impact the cantilever dynamics substantially.
The distinction between the corresponding work functions was anticipated to produce a CPD when an n-type Si tip was placed close to a conductive surface. Experiments were conducted as a function of laser power and tip bias voltage to examine the effect of laser illumination on the CPD value.
Photothermal and Photovoltaic Effects in Atomic Force Microscopy Probes
The current paper demonstrated the modifications induced by the photothermal and photovoltaic effects on the phase dynamics of commercially available atomic force microscopy probes in vacuum conditions. The authors discovered a significant photothermal effect and photovoltage generation at all tip apexes of Si by examining the oscillation phase in micromechanical oscillators as a function of the applied voltage.
The parabolic phase signal was displaced by the photothermal and photovoltaic effects on the voltage or phase axes. The absence of the observed displacement caused by the photovoltage on Au-coated probes or when the laser was directed at the cantilever confirmed the semiconductor nature and the band bending in the Si apexes.
For each of the studied tips, quantifying the phase shift produced by the photothermal effect exhibited linear fluctuations with laser intensity. When the laser was directed on the Si tip or the Si cantilever containing the tip, the slopes of the linear deviations were discovered to be identical, indicating good thermal dissipation along the cantilever body. The slope was around three times smaller in the case of an Au-coated probe, indicating less heat generation and light absorption.
The authors believe that the results could be applied to the entire spectrum of atomic force microscopy methods that use visible light as an additional instrument for local optical characterization.
M.D. Pichois, X. Henning, M.A. Hurier, et al. (2022) Photovoltaic and Photothermal Effects Induced by Visible Laser Radiation in Atomic Force Microscopy Probes. Ultramicroscopy. https://www.sciencedirect.com/science/article/pii/S0304399122001206