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Well-ordered nanoparticle arrays are attractive platforms for a variety of analytical applications, but the fabrication of such arrays is generally challenging. Here, it is demonstrated that scanning electrochemical cell microscopy (SECCM) can be used as a powerful, instantly reconfigurable tool for the fabrication of ordered nanoparticle arrays. Using SECCM, Ag nanoparticle arrays were straightforwardly fabricated via electrodeposition at the interface between a substrate electrode and an electrolyte-filled pipet. By dynamically monitoring the currents flowing in an SECCM cell, individual nucleation and growth events could be detected and controlled to yield individual nanoparticles of controlled size. Characterization of the resulting arrays demonstrate that this SECCM-based approach enables spatial control of nanoparticle location comparable with the terminal diameter of the pipet employed and straightforward control over the volume of material deposited at each site within an array. These results provide further evidence for the utility of probe-based electrochemical techniques such as SECCM as tools for surface modification in addition to analysis. 

Abstract Transition metal dichalcogenide (TMD) heterostructures are promising for a variety of applications in photovoltaics and photosensing. Successfully exploiting these heterostructures will require an understanding of their layer-dependent electronic structures. However, there is no experimental data demonstrating the layer-number dependence of photovoltaic effects (PVEs) in vertical TMD heterojunctions. Here, by combining scanning electrochemical cell microscopy (SECCM) with optical probes, we report the first layer-dependence of photocurrents in WSe 2 /WS 2 vertical heterostructures as well as in pristine WS 2 and WSe 2 layers. For WS 2 , we find that photocurrents increase with increasing layer thickness, whereas for WSe 2 the layer dependence is more complex and depends on both the layer number and applied bias ( V b ). We further find that photocurrents in the WSe 2 /WS 2 heterostructures exhibit anomalous layer and material-type dependent behaviors. Our results advance the understanding of photoresponse in atomically thin WSe 2 /WS 2 heterostructures and pave the way to novel nanoelectronic and optoelectronic devices. 

Two-dimensional semiconductors (2DSCs) are attractive for a variety of optoelectronic and catalytic applications due to their ability to be fabricated as wide-area, monolayer-thick films and their unique optical and electronic properties which emerge at this scale. One important class of 2DSCs are the transition metal dichalcogenides (TMDs), which are of particular interest as absorbing layers in ultrathin optoelectronic devices. While TMDs are known to exhibit excellent photovoltaic properties at the bulk level, it is not yet clear how carriers are transported in these materials at thicknesses approaching the monolayer limit, where distinct changes in band structure and the nature of photogenerated carriers occur. Here, it is demonstrated that electrochemical microscopy techniques can be employed as powerful tools for visualizing these processes in 2DSCs, even within individual monolayers. Carrier generation-tip collection scanning electrochemical cell microscopy (CG-TC SECCM), which utilizes spatially-offset optical and pipet-based electrochemical probes to locally generate and detect photogenerated carriers, was applied to visualize carrier generation and transport within well-defined n-WSe 2 samples prepared via mechanical exfoliation. Data from these experiments directly reveal how carrier transport varies within complex 2DSC structures as layer thicknesses approach the monolayer limit. These results not only provide valuable new insights into carrier transport within monolayer TMD materials, but also demonstrate electrochemical imaging to be a powerful, yet underutilized approach for visualizing solid-state processes in semiconducting materials.