Single-dot tunneling spectroscopy of CdSe colloidal nano-dot by atomic force microscopy

近年来，高度单分散的胶体半导体纳米点由于具有独特的光学性质而引起了人们的广泛关注。我们一直对胶体纳米点的电学性质感兴趣，因为它们似乎对解释光学性质很重要。具体地说，界面能量状态可能与诸如点的光记忆效应的非线性光学性质相关。因此，研究胶体纳米点的电流-电压(I-V)特性对于利用隧道光谱揭示胶体纳米点的能级具有重要意义。为了研究单个胶体纳米点的电子输运，我们使用了导电尖端原子力显微镜。(AFM)我们发现，即使在室温下，我们也能用电导针尖AFM测量单个胶体纳米点的伏安特性。然而，我们发现用接触式原子力显微镜测得的网点高度远小于平均d…。从光吸收测量中估计的网点的更多长度。我们推测，当金属涂层的硅尖接触到接触式AFM测量时，这些点的位置发生了轻微的变化。这种位置波动似乎是由于针尖和样品之间的巨大毛细管力引起的，从而阻碍了我们对点高度和通过点的电流的准确测量。因此，我们采用导电碳纳米管(CNT)针尖来进行CdSe胶体纳米点的导电针尖AFM，发现碳纳米管针尖与样品之间的毛细管力减少到金属涂层硅尖的三分之一左右，并且点高的测量精度得到了很大的提高。结果表明，在室温下获得了单个胶体纳米点的I-V特性，而不需要金属涂层硅尖所需要的额外的点锚定过程。在I-V曲线上，观察到了明显的电导变化，这表明电子在纳米点中发生了共振隧穿。此外，为了在导电针尖AFM测量过程中稳定纳米点的位置，还研究了由胶体纳米点组成的二维岛的形成。我们采用水平剥离工艺在云母上蒸发的Au(111)薄膜上制备了单点高度的胶体纳米点薄膜，并用接触式原子力显微镜证实了100 nm尺度的二维点岛的存在。将对这些岛屿进行I-V测量。较少

Recently, highly monodisperse colloidal semiconductor nano-dots have been attracting much attention since they exhibit unique optical properties. We have been interested in the electronic properties of colloidal nano-dots since they seem to be important to explain optical ones. In particular, the interface energy states are likely correlated with non-linear optical properties such as optical memory effect of the dots. It is therefore important to investigate the current-voltage (I-V) characteristics of the colloidal nano-dots in order to reveal the energy levels of the dots by tunneling spectroscopy.For the study of the electron transport through single CdSe colloidal nanodots, we have employed conductive-tip atomic force microscopy. (AFM) We have shown that current-voltage characteristics of single CdSe colloidal nanodot were measured even at room temperature by conductive-tip AFM. However, we found that the measured dot heights by contact-mode AFM were much smaller than the average d … More iameter of the dots estimated from optical absorption measurement. We speculated that the dots slightly changed their positions when the metal-coated Si tips came in contact for contact-mode AFM measurements. This position fluctuation seemed to be caused by the large capillary force between the tip and sample, and prevents us from accurate measurement of the dot height and the current through the dot.We therefore adopted conductive carbon nano-tube(CNT) tips for conductive-tip AFM of CdSe colloidal nano-dots, and found that the capillary force between the CNT tip and sample is reduced to be about one thirtieth of that for the metal-coated Si tips, and that the dot height measurement is much improved in accuracy. As a result, I-V characteristics of single colloidal nano-dots have been obtained at room temperature without extra dot anchoring process which was required for metal-coated Si tips. In the I-V curves, clear conductance changes were observed which suggested resonant electron tunneling through the nano-dot.Also, the formation of two-dimensional islands consist of colloidal nano-dots was investigated in order to stabilize the dot position during conductive-tip AFM measurements. We fabricated single-dot height films of the colloidal nano-dots by horizontal lift-off process on Au (111) films evaporated on mica substrates, and confirmed the existence of 100-nm-scale two-dimensional islands of the dots by contact-mode AFM. I-V measurements of these islands will be performed. Less