Facile Fabrication of Spherical Nanoparticle-Tipped AFM Probes for Plasmonic Applications

decade, [ 15 ] beginning with the use of fi bers as mounting structures [ 16–19 ] and progressing onto the use of scanning probe microscopy (SPM) tips. [ 6,8,13,20,21 ] While mounting nanoparticles onto SPM tips is more diffi cult than with fi bers the additional capabilities of the SPM tip have made such tips desirable. However , these tips typically require complicated assembly processes to precisely secure a single nanoparticle at the apex of the tip, greatly increasing their fabrication time and costs. More recent methods have attempted to address the complexity issue by directly depositing nanoparticles onto the apex by exploiting localized chemical reactions. However, these techniques have still been limited by cost or required specialist equipment, [ 18,20 ] incompatibility with SPM probes, [ 7,17 ] or constraints on nanoparticle growth either in size [ 22 ] or material. [ 6 ] It remains a challenge to fi nd a simple and effi cient method for reliably producing spherical nanoparticle tips. Here, we provide a simple and effi cient method for reliably producing spherical nanopar-ticle tips using apex-selected electrochemical growth. Electrochemical deposition is highly suited to the tip geometry because of the large fi eld enhancement localized at the sharp point. Due to the signifi cantly reduced radius of curvature at the tip apex, the equipotential surface resulting from an applied voltage leads to compression of fi eld lines in the region. This strongly increases the fi eld amplitude at the tip apex and is known as the lightning rod effect. Under such conditions, the rate of electrochemical reactions is signifi cantly increased around the vicinity of the tip apex. By exploiting this localized fi eld enhancement, it is possible to grow a spherical nanoparticle directly onto the tip apex. While use of the lightning rod effect for electrochemical growth has been used to grow dense forests of nanoparticles [ 23,24 ] it has yet to be applied to the fabrication of single nanoparticles at the tips apex. A disadvantage of elec-trochemical deposition is that it is diffi cult to selectively grow nanoparticles on the tips. Here by using single-pulse high-fi eld electrochemical growth, we selectively grow nanoparticles on the tip apex. Using this approach, we demonstrate an effi cient, high-throughput technique for reliably producing metallic spherical nanoparticle tips using only a simple electrochemical cell. Fabrication of Au spherical nanoparticle (AuNP) tips onto commercial AFM probes …