Department of Chemical Engineering, Massachusetts Institute of Technology , Cambridge, Massachusetts 02139, United States.
Physics Science and Engineering Division, King Abdullah University of Science and Technology , Thuwal 23955-6900, Kingdom of Saudi Arabia.
Nano Lett. 2016 Jun 8;16(6):3571-7. doi: 10.1021/acs.nanolett.6b00699. Epub 2016 May 4.
In the pursuit of two-dimensional (2D) materials beyond graphene, enormous advances have been made in exploring the exciting and useful properties of transition metal dichalcogenides (TMDCs), such as a permanent band gap in the visible range and the transition from indirect to direct band gap due to 2D quantum confinement, and their potential for a wide range of device applications. In particular, recent success in the synthesis of seamless monolayer lateral heterostructures of different TMDCs via chemical vapor deposition methods has provided an effective solution to producing an in-plane p-n junction, which is a critical component in electronic and optoelectronic device applications. However, spatial variation of the electronic and optoelectonic properties of the synthesized heterojunction crystals throughout the homogeneous as well as the lateral junction region and the charge carrier transport behavior at their nanoscale junctions with metals remain unaddressed. In this work, we use photocurrent spectral atomic force microscopy to image the current and photocurrent generated between a biased PtIr tip and a monolayer WSe2-MoS2 lateral heterostructure. Current measurements in the dark in both forward and reverse bias reveal an opposite characteristic diode behavior for WSe2 and MoS2, owing to the formation of a Schottky barrier of dissimilar properties. Notably, by changing the polarity and magnitude of the tip voltage applied, pixels that show the photoresponse of the heterostructure are observed to be selectively switched on and off, allowing for the realization of a hyper-resolution array of the switchable photodiode pixels. This experimental approach has significant implications toward the development of novel optoelectronic technologies for regioselective photodetection and imaging at nanoscale resolutions. Comparative 2D Fourier analysis of physical height and current images shows high spatial frequency variations in substrate/MoS2 (or WSe2) contact that exceed the frequencies imposed by the underlying substrates. These results should provide important insights in the design and understanding of electronic and optoelectronic devices based on quantum confined atomically thin 2D lateral heterostructures.
在追求超越石墨烯的二维（2D）材料的过程中，人们在探索过渡金属二卤化物（TMDC）的令人兴奋和有用的特性方面取得了巨大的进展，例如在可见光范围内的永久带隙以及由于 2D 量子限制而从间接带隙到直接带隙的转变，以及它们在广泛的器件应用中的潜力。特别是，通过化学气相沉积方法合成不同 TMDC 的无缝单层横向异质结构的最新成功，为产生平面 p-n 结提供了有效解决方案，这是电子和光电设备应用中的关键组件。然而，合成异质结晶体在同质以及横向结区域的电子和光电特性的空间变化，以及在与金属的纳米尺度结处的电荷载流子输运行为仍然没有得到解决。在这项工作中，我们使用光电流光谱原子力显微镜来成像偏置 PtIr 尖端和单层 WSe2-MoS2 横向异质结构之间产生的电流和光电流。在正向和反向偏压下在黑暗中进行的电流测量揭示了 WSe2 和 MoS2 的相反特性二极管行为，这是由于形成了具有不同特性的肖特基势垒。值得注意的是，通过改变施加尖端电压的极性和大小，可以观察到显示异质结构光响应的像素被选择性地打开和关闭，从而实现了可切换光电二极管像素的超分辨率阵列。这种实验方法对于在纳米分辨率下进行区域选择性光电探测和成像的新型光电技术的发展具有重要意义。物理高度和电流图像的比较 2D 傅里叶分析表明，衬底/MoS2（或 WSe2）接触处的高空间频率变化超过了基础衬底施加的频率。这些结果应该为基于量子限制原子薄 2D 横向异质结构的电子和光电设备的设计和理解提供重要的见解。
