The Influences Of Thermal Annealing On The Gold Nanoparticle Ultrathin Film’s Structure And Photoelectron Emission

It is well known that Au is not an ideal material to emit electrons due to its quite low Fermi level below the vacuum level.However,it has been reported that alkanethiols notably enhance the photoelectron emission of the Au nanoparticles(NPs)ligated by the alkanethiols.Thermal annealing can regulate the structure and morphology of the Au NP films as well as even the Au NP size,which thus modifies the optical and electric properties of the Au-NP films significantly.Nevertheless,little is known about the influences of the thermal annealing on the photoelectron emission from the Au-NP film,which is necessary to be investigated for a better understanding of the mechanism for the enhanced photoelectron emission from the Au-NP films.Thus,the main work done in this thesis is as the following:1.The size-controlled synthesis of 11-Mercaptoundecanoic Acid-capped Au Nanoparticles with a narrow size distribution using a modified two-phase method.(1)We used a modified one-step two-phase method to synthesize 11-Mercaptoundecanoic MUA-capped Au NPs.The effects of the gold-to-thiol reactant molar ratio(Au/S RMR)on the size and structure of the Au NPs were investigated.The synthesized Au NP size is uniform with a narrow distribution.The surfaces of Au NPs were covalently bonded by 11-mercaptoundecanoic acid via Au-S bonds as evidence by FT-IR spectroscopy.The XRD pointed out that the Au NPs are with a face-centered cubic crystal structure.Due to the reactive nature of the functional-COOH terminals of these Au NPs with a uniform controllable size,they would be potentially applied in the fields such as biological sensor,self-assembly,optical and electronic devices.(2)A two step-two phase method was adopted to make MUA capped Au NP with a large size over 5 nm,and their stability was also investigated.2.Enhanced Photoelectron Emission from Ultra-thin Au NP Film by the thermal annealingMUA ligated Au NPs with an average diameter of ~1.4 nm were used to fabricate ultrathin film with a thickness of about 10 nm.AFM and SEM showed that this film is quite smooth,and AuNP are distributed uniformly in the film.Using SEM,it was found that S composition decreases with increasing annealing temperature.UPS(Ultraviolet Photoelectron Spectroscopy)revealed that the secondary electron photoemission peak from Au NP film after being annealed at 150 °C becomes about 4 times larger than that without annealing.However,an annealing at an even higher temperature caused a dramatic decrease of secondary electron photoemission peak.Conclusion: The present study demonstrated that an annealing at a proper elevated temperature enhanced the secondary electron photoemission from ultrathin MUA ligated Au NP films,which would promote applications of the ligated Au NP films as promising photoelectronemission materials for electron sources or photon detectors.