Justin R. Caram, University of California, Los Angeles
Mesoscale Semiconductor Nanoplatelets
Written by Rosemary Calabro
Detectors that operate in the short-wave infrared window (SWIR) are highly desirable for applications ranging from fiberoptics and communications to bioimaging but present several challenges in their design and fabrication. Unfortunately, many detectors that operate in the SWIR range are highly expensive and lack the scalability that silicon-based detectors have because there are not many materials that operate in this wavelength range. Justin Caram proposed colloidal nanocrystals as a solution to this problem because they are scalable, processable, and can be quantum confined in the wavelengths of interest. Specifically, type II–VI semiconductor nanoplatelets are promising because they can reach large lateral sizes and quantum confinement is possible by controlling the layer thickness. They can be colloidally grown with control over the number of layers, have narrow ensemble linewidths, and large absorption cross sections. Caram showcased some of his group’s work that optimized the formation of various nanoplatelets for SWIR devices. They used a cation exchange strategy and a seeded growth strategy to form nanoplatelets in extended size ranges. They aimed to increase the lateral dimensions because the current sizes that could be achieved were too small for the target applications. They explored two strategies to improve these properties. First, they explored parameters that could encourage ripening of the nanoplatelets which allowed dissolution of smaller nanoplatelets and growth onto larger ones. They then also explored strategies to optimize the slow-injection rate used during synthesis. By exploring parameters such as temperature, seed concentration, and injection rate the researchers were able to optimize the synthesis to achieve larger sizes. They further increased the sizes by considering parameters such as capping ligands and how to minimize the strain they can induce on the nanoplatelets which can limit their size and promote formation of nanocrystals. Caram also addressed a challenge they encountered with oxide formation from metal precursors which limited the growth. Caram closed his talk by showing some impressive wide field microscopy images that demonstrated the absorption and emission properties of the nanoplatelets, something that is not possible to achieve unless the platelets are quite large. This talk highlighted the potential of nanoplatelets for reaching SWIR wavelength ranges and implementations into devices.