Micro raman spectrometer system

For structural and elemental analyses of the WSe₂/WCL/h-BN and MoS₂/WCL/h-BN regions, X-TEM (JEM ARM 200 F) and EELS (GIF Quantum ER, 200 keV) measurements were performed. Raman analysis was performed at various positions on the WSe₂/WCL/h-BN and MoS₂/WCL/h-BN samples using a WITec micro-Raman spectrometer system with a frequency-doubled Nd-doped yttrium aluminum garnet (Nd-YAG) laser beam (532-nm laser excitation). AFM was performed using an NX10 system (Park Systems Corp.). Electrical measurements of the synaptic devices were performed using an HP-4155A semiconductor parameter analyzer connected to a voltage spike generator (Keysight, 33500B). The aforementioned measurement setup for the synaptic devices is described in detail in Supplementary Fig. 2.

Raman spectroscopy analysis was performed in the HfS₂, pentacene, and HfS₂/pentacene overlapped regions using a WITec micro‐Raman spectrometer system with a frequency‐doubled neodymium‐doped yttrium aluminum garnet (Nd–YAG) laser beam (532 nm laser excitation). Additionally, AFM was conducted in the HfS₂/pentacene heterojunction region using an NX10 (Park Systems Corp.). To investigate the core levels of the HfS₂ and pentacene materials, XPS information was extracted using a MultiLab 2000 (Thermo VG, Mg Kα source) after the peak energies were calibrated with respect to the C 1s peak at 284.5 eV. A depth profiling analysis was conducted in the small‐area XPS mode, where a 3 mm × 3 mm area was etched for 30 s using an in situ Ar⁺‐ion gun. The electrical properties of the HfS₂/pentacene NDR device were investigated under vacuum conditions at room temperature by using a semiconductor parameter analyzer (Keithley 4200) connected to a probe station.

Raman studies were conducted using a WITec micro-Raman spectrometer system with a frequency-doubled Nd-YAG laser beam (532 nm laser excitation). The atomic force microscope analysis was carried out in an XE 100 (Park Systems Corp.) system. The electrical transport measurements were conducted at room temperature under ambient conditions in a probe station with a Keysight B2912A. The temperature-dependent electrical characteristics were measured in a vacuum chamber (below 10⁻⁴ Torr) using a Keithley 4200 Semiconductor Parameter Analyzer. The KPFM measurement was performed using NTEGRA Spectra (NT-MDT).