Optical properties and thermal conductivity of resistive Au nanogranular films
Networks based on nanometer-sized resistively switching junctions are considered promising elements for the fabrication of neuromorphic computing architectures. Such architectures are intended for data-intensive processing tasks in a way similar to synapse networks. Resistive switching refers to the reversible change of resistance of a dielectric material in response to the application of a strong external electric field. Networks of gold clusters, fabricated by supersonic cluster beam deposition, are ohmic conductors when deposited but can be electrically activated to a state where their conduction behaviour is no longer linear, and where they can be reproducibly switched between low resistance and high resistance states. At room temperature and atmospheric pressure, stability and reproducibility of the resistance switching of such nanogranular Au films was observed to last over many hours thus making them ideal test beds for exploration of the basic mechanisms of the switching processes and allow convenient fabrication of devices that may have neuromorphic properties. The objective of this PhD project was to study other properties of these same nanogranular gold films, namely their linear and nonlinear optical properties and their thermal conductivity. The linear optical properties were observed using femtosecond transient absorption spectroscopy (fTAS) and their film morphology was explored with atomic force microscopy. The switch activation mechanisms that transformed the nanogranular Au film into a resistively switchable one, clearly influenced the plasmonic effects. This implies that the plasmonic effects of the nanogranular Au film can be tuned using resistive switching and since strong plasmonic effects enhance light absorption, such films may be used for optical switching applications and modulation of optical signals in artificial optoelectronic synapses that have been engineered plasmonically. As a result of switch activation and resistive switching, structural changes of the film were observed using atomic force microscopy and are reflected in the fTAS measurements, The film morphology change could be inferred from a higher entropy value of the images of the switched film. This higher entropy value can be correlated to the transient absorption signals that showed a variation in the electron-phonon relaxation dynamics with each resistance state (LOW and HIGH resistance states), and a modulation of the plasmon features in different spectral regions. The electron-phonon dynamics observed using fTAS, thus help to understand both LOW and HIGH resistance states at the microscopic level. Despite the efficient energy transfer in the switched nanogranular Au film in comparison to the pristine film, as deduced from the longer relaxation time for the former (section 3.4), the strong electron-phonon coupling effects mean that there are more scattering sites, which negatively affect the thermal conductivity of the switched nanogranular Au film. As such, to improve the performance of the resistive switch-activated nanogranular Au film for practical neuromorphic devices, the fabrication of the nanogranular films should be tailored in such a way as to reduce the scattering sites of the film, thus improving the thermal conductivity of the film. Whether this can be done without compromising the nonlinear electrical conductivity remains to be explored. Nonlinear optical properties of the as-deposited and activated nanogranular gold films that are crucial for neuromorphic application were measured using Z-scan and optical Kerr effect (OKE) techniques. The effect of switch activation is reflected as microscopical structural changes in the nanogranular Au films as revealed by a high entropy value of the scanning electron microscopy images and by the variation of the distribution of the widths of the connected clusters between the pristine and activated film. As a result of switch activation, the third-order nonlinearity of the nanogranular Au films was enhanced. This property of the switch-activated films can be used for modulation of light intensity and polarisation for complex nonlinear activation functions in neuromorphic systems. The nanogranular Au films that exhibited surface plasmon resonance, namely the 5 nm thick as-deposited and the switch-activated 10 nm thick films, also showed higher values for the nonlinear susceptibility. Z-scan technique measurements showed that switch activation affected the nonlinear absorption and nonlinear refractive index of the activated nanogranular Au films. Enhanced nonlinear absorption effects can be useful in memory and optical switching applications. An increase in the nonlinear refractive index is essential for creating bi-stability and multi-stability for optical neurons and synapses (the basic neuromorphic computing elements). The thermal properties of a 50 nm thick nanogranular Au film were further examined using ultrafast electron diffraction (UED). Upon laser photoexcitation of the 50 nm films, energy is first absorbed by (nearly) free electrons, followed by transfer to the lattice via electron-lattice collisions. This process results in an increase in lattice temperature, detected experimentally by the rise in atomic mean-square displacement. The electron-phonon coupling constant was calculated to be 7.67 x 10 16 W m-3K, which is higher than typical values reported in the literature. This stronger coupling implies increased electron-phonon scattering, reducing the mean free paths of both electrons and phonons, and thereby lowering the thermal conductivity. Furthermore, the diffraction intensity decay, described using the Debye-Waller model, exhibited a slow decay with a time constant of 8.5 ± 0.7 ps, further indicating inefficient heat dissipation. These findings suggest that optimizing the grain size in nanogranular Au films could be crucial for improving thermal performance. Increasing grain size could reduce scattering sites and enhance thermal conductivity—an essential factor for practical neuromorphic device applications. However, also this optimisation has to be tested for compatibility with the nonlinear electrical conductivity. In conclusion, this PhD project demonstrated that switch activation and resistive switching not only modifies the electrical behaviour of nanogranular gold films but also significantly impacts their optical and thermal properties. Microscopy methods (AFM, SEM) provided conclusive evidence of structural changes associated with switch activation and switching. The transition to a more complex, more connected cluster network influences both plasmonic effects and energy relaxation dynamics as shown with optical spectroscopic techniques (fTAS, Z-scan, OKE). However, the enhanced electron-phonon coupling, while beneficial for optical performance, hinders thermal conductivity. Therefore, future efforts should focus on optimizing film morphology—particularly grain size and connectivity—to balance optical performance with thermal management, without compromising the electronic properties.
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