Self-Assembly of CdTe Tetrapods into Network Monolayers at the Air/Water Interface

Abstract

Cadmium telluride (CdTe) tetrapods are synthesized with varying aspect ratios through multiple injections of the Te precursor, which provides an excellent means of controlling and tailoring the optical properties of the tetrapods. The self-assembly of CdTe tetrapods at the air/water interface is explored using the LangmuirBlodgett (LB) technique due to potential use in solar cells arising from the intriguing tetrapod shape that improves charge transport and the optimum band gap energy of CdTe that enhances light absorption. Interestingly, the Langmuir isotherm shows two pressure plateau regions: one at ∼10 mN/m with the other at the high surface pressure of ∼39 mN/m. LB deposition at various pressures allows the discernment of the unique two-dimensional packing alluded in the isotherm. By placing CdTe at the air/water interface, it is revealed in the deposition that the tetrapods experienced a dewetting phenomenon, forming a ribbon structure at the onset of surface pressure with a height corresponding to the length of one tetrapod arm. With the increase of surface pressure, the ribbons widen to an eventual large-scale percolated network pattern. The packing density of tetrapods is successfully manipulated by controlling the surface pressure, which may find promising applications in optoelectronic devices. KeywordsMaterials Science and Engineering, CdTe, Langmuir-Blodgett, network, self-assembly, tetrapod Disciplines Biochemical and Biomolecular Engineering | Chemical Engineering | Materials Science and Engineering CommentsReprinted with permission from ACS Nano 4 (2010) Semiconductor nanocrystals with various shapes have been synthesized, with quantum dot (QD), quantum rod (QR; i.e., nanorod), tetrapod, and nanowire synthesis being the most refined and controlled. While QDs are relatively easy to synthesize, properties that are more advantageous can be realized with the unique shapes. For example, the ability to manipulate the shape of nanocrystals has led to QRs with diameters ranging from 2 to 10 nm and lengths ranging from 5 to 100 nm. 17,18 Because of their intrinsic structural anisotropy, QRs possess many unique properties that make them potentially better nanocrystals for photovoltaic cells and biomedical applications than QDs. They possess the polarized emission and their Stokes shift is dependent upon their aspect ratio. 17,18 Photovoltaic cells made of QRs and conjugated polymers show an improved optical absorption in the red and near-infrared ranges originating from the QRs.3 Moreover, the long axis of the QRs provides continuous paths for transporting electrons. The performance of photovoltaic cells can be further improved if the QRs are vertically aligned between two electrodes to minimize the carrier transport pathways. 3 The elongated nanorods and branched nanocrystals (i.e., tetrapods) enable more effective charge transport.To date, there are few reports of nanocrystal assembly at the air/water interface.19,20 By contrast, studies have been conducted at the fluid/fluid interface, with ...

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