Matsuyama Eiji, Kondo Takahiro, Oigawa Haruhiro, Guo Donghui, Nemoto Shojiro, Nakamura Junji
Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki, 305-8573 Japan.
Sci Rep. 2014 Oct 24;4:6711. doi: 10.1038/srep06711.
Detection of the extremely weak signals in spectroscopy over an extremely wide frequency region is central to diverse sciences, including materials science, biology, astronomy and chemistry. Here we show a new type of atomic-scale spectroscopy, heterodyne scanning tunnelling spectroscopy (HSTS), which is based on the innovative application of the nonlinear heterodyne-mixing detection at the metal-insulator-metal (MIM) heterojunction of STM tip-vacuum-sample. The principle of HSTS is identical to that of the Atacama Large Millimeter Array (ALMA) space telescope in terms of using heterojunction for detecting extremely weak signals by converting from terahertz region to lower frequency regions. The MIM detector of ALMA, which is composed of niobium-titanium-nitride (NbTiN) tip-insulator-NbTiN, is very similar in shape and size to that of HSTS. We successfully detect a heterodyne beat signal f₃(= |f₂- f₁) and intermodulation distortion via tunnelling current by superimposing two different AC signals, f₁ and f₂, onto the DC tunnelling current at a highly oriented pyrolytic graphite (HOPG) surface. We then obtain spectra of the localized electronic states of HOPG by using f₃. HSTS can be performed with a high resolution and over a wide energy range, including the terahertz range.
在极宽频率范围内检测光谱中的极微弱信号是包括材料科学、生物学、天文学和化学在内的多种科学的核心。在此，我们展示了一种新型的原子尺度光谱技术，外差扫描隧道光谱（HSTS），它基于在扫描隧道显微镜（STM）针尖 - 真空 - 样品的金属 - 绝缘体 - 金属（MIM）异质结处对非线性外差混频检测的创新应用。HSTS的原理与阿塔卡马大型毫米波阵列（ALMA）空间望远镜的原理相同，即在通过从太赫兹区域转换到较低频率区域来利用异质结检测极微弱信号方面。ALMA的MIM探测器由氮化铌钛（NbTiN）针尖 - 绝缘体 - NbTiN组成，其形状和尺寸与HSTS的非常相似。我们通过在高度定向热解石墨（HOPG）表面将两个不同的交流信号f₁和f₂叠加到直流隧道电流上，成功地通过隧道电流检测到了外差拍频信号f₃（= |f₂ - f₁|）和互调失真。然后，我们利用f₃获得了HOPG的局域电子态光谱。HSTS可以在包括太赫兹范围在内的宽能量范围内以高分辨率进行。
