Tuning Electrical and Optoelectronic Properties of Single Cadmium Telluride Nanoribbon

Cadmium telluride nanoribbons were synthesized electrochemically and formed into nanodevices using lithographically patterned nanowire electrodeposition (LPNE) that integrated synthesis and device fabrication together. By controlling the composition of the CdTe nanoribbons via adjusting the electrodeposition potential, electrical properties (i.e., electrical resistivity and field-effect transistor (FET) mobility) and optoelectronic property (photocurrent) were tuned and compared. Electrical resistivity of CdTe nanoribbons was strongly dependent on the Te content as higher Te content provided higher carrier concentration. Electrical conduction was limited by grain boundary traps due to polycrystalline structure. FET mobility was correlated to electrical resistivity and photocurrent, and annealed samples showed improved FET mobility, electrical resistivity, and photocurrent response. ■ INTRODUCTION Cadmium Telluride (CdTe) is an important II−VI compound semiconductor because it has a direct band gap of 1.44 eV at room temperature with a high optical absorption coefficient in the visible spectrum enabling it to be a great candidate material for photovoltaics. Indeed, CdTe thin film based solar cells have been investigated by many researchers over the past decades and they are recently becoming commercially available. Onedimensional nanomaterials, such as nanowires, nanoribbons, and nanotubes, have been researched extensively because of their unique size dependent optical, electrical, and magnetic properties with potential application in nanoscale electronic, optoelectronic, spintronics, photovoltaics, thermoelectrics, and sensors. Current research efforts have been focused on controlling the synthesis and integrating these nanoscale building blocks into high density complex devices. CdTe nanowires have been synthesized by a variety of techniques, including self-assembly from CdTe nanoparticles, solutionbased chemical synthesis, and template-directed electrodeposition. Although the synthesis of these CdTe nanowires have been demonstrated by various groups by a multitude of techniques, there exist very few reports on the electrical and optoelectronic properties of a single nanostructure based device. We have previously reported electrical and optoelectronic properties of a single CdTe nanowire for the first time. We were able to synthesize CdTe nanowires with controlled composition and dimension using template directed electrodeposition; however, the fabrication yield of nanodevices with good electrical contact were very low, limiting our ability to investigate systematically the size and composition dependent electrical and optoelectronic properties. A novel nanofabrication technique with a significant improvement over existing template-directed electrodeposition followed by device assembly known as lithographically patterned nanowire electrodeposition (LPNE) has been developed by the Penner group. It retains many advantages of template-directed electrodeposition with a few improvements. One important advance of this technique is that it integrates both the bottom-up electrochemical synthesis of the nanomaterials and the top-down photolithography to fabricate batches of high density nanodevices with desired size and composition along with integrated microelectrodes. Typical nanodevice fabrication involves a batch synthesis of these nanowires followed by the assembly of the nanowires into nanodevices on a substrate using techniques such as e-beam lithography, focus ion beam (FIB), AC dielectrophoretic, and magnetic assembly, which are low-yield and costly. Another advantage of LPNE is that it allows precise location and orientation of the nanodevices on a wafer level improving the yields of functional devices, which is extremely attractive for mass production. Our group has also implemented this technique to fabricate Bi2Te3 and polypyrrole nanoribbon electrical nanodevices for thermoelectric and gas sensing applications. Single CdTe nanoribbon based devices were fabricated using LPNE technique, and their electrical and optoelectronic properties were shown to be tunable by the composition of the CdTe nanoribbons. Electrodeposition potential dictated the Received: January 3, 2012 Revised: April 3, 2012 Published: April 4, 2012 Article