King Benjamin, Panchapakesan Balaji
Small Systems Laboratory, Department of Mechanical Engineering, University of Louisville, Louisville, KY 40292, USA.
Nanotechnology. 2014 May 2;25(17):175201. doi: 10.1088/0957-4484/25/17/175201. Epub 2014 Apr 10.
In this paper, we report ultra-thin liquid crystal films of semiconducting carbon nanotubes using a simple vacuum filtration process. Vacuum filtration of nanotubes in aqueous surfactant solution formed nematic domains on the filter membrane surface and exhibited local ordering. A 2D fast Fourier transform was used to calculate the order parameters from scanning electron microscopy images. The order parameter was observed to be sensitive to the filtration time demonstrating different regions of transformation namely nucleation of nematic domains, nanotube accumulation and large domain growth.Transmittance versus sheet resistance measurements of such films resulted in optical to dc conductivity of σ(opt)/σ(dc) = 9.01 indicative of purely semiconducting nanotube liquid crystal network.Thin films of nanotube liquid crystals with order parameters ranging from S = 0.1-0.5 were patterned into conducting channels of transistor devices which showed high I(on)/I(off) ratios from 10-19,800 and electron mobility values μ(e) = 0.3-78.8 cm(2) (V-s)(-1), hole mobility values μ(h) = 0.4-287 cm(2) (V-s)(-1). High I on/I off ratios were observed at low order parameters and film mass. A Schottky barrier transistor model is consistent with the observed transistor characteristics. Electron and hole mobilities were seen to increase with order parameters and carbon nanotube mass fractions. A fundamental tradeoff between decreasing on/off ratio and increasing mobility with increasing nanotube film mass and order parameter is therefore concluded. Increase in order parameters of nanotubes liquid crystals improved the electronic transport properties as witnessed by the increase in σ(dc)/σ(opt) values on macroscopic films and high mobilities in microscopic transistors. Liquid crystal networks of semiconducting nanotubes as demonstrated here are simple to fabricate, transparent, scalable and could find wide ranging device applications.
在本文中，我们报道了使用简单的真空过滤工艺制备的半导体碳纳米管超薄液晶薄膜。在水性表面活性剂溶液中对纳米管进行真空过滤，在滤膜表面形成了向列畴，并呈现出局部有序性。利用二维快速傅里叶变换从扫描电子显微镜图像计算有序参数。观察到有序参数对过滤时间敏感，表明存在不同的转变区域，即向列畴的成核、纳米管的堆积和大畴的生长。对这类薄膜的透过率与薄层电阻进行测量，得到光学电导率与直流电导率之比σ(opt)/σ(dc) = 9.01，表明是纯半导体纳米管液晶网络。将有序参数范围为S = 0.1 - 0.5的纳米管液晶薄膜图案化为晶体管器件的导电通道，这些通道显示出10 - 19,800的高I(on)/I(off)比，电子迁移率值μ(e) = 0.3 - 78.8 cm²(V - s)⁻¹，空穴迁移率值μ(h) = 0.4 - 287 cm²(V - s)⁻¹。在低有序参数和薄膜质量下观察到高I on/I off比。肖特基势垒晶体管模型与观察到的晶体管特性一致。电子和空穴迁移率随有序参数和碳纳米管质量分数的增加而增加。因此可以得出结论，随着纳米管薄膜质量和有序参数的增加，开/关比降低与迁移率增加之间存在基本的权衡。纳米管液晶有序参数的增加改善了电子传输性能，这在宏观薄膜上σ(dc)/σ(opt)值的增加以及微观晶体管中的高迁移率中得到了证明。本文所展示的半导体纳米管液晶网络易于制造、透明、可扩展，并且可以找到广泛的器件应用。
