Few-Hundred GHz Carbon Nanotube Nanoelectromechanical Systems (NEMS)

We study 23–30 nm long suspended single-wall carbon nanotube quantum dots and observe both their stretching and bending vibrational modes. We use low-temperature DC electron transport to excite and measure the tubes’ bending mode by making use of a positive feedback mechanism between their vibrations and the tunneling electrons. In these nanoelectromechanical systems (NEMS), we measure fundamental bending frequencies <i>f</i>_bend ≈ 75–280 GHz and extract quality factors <i>Q</i> ∼ 10⁶. The NEMS's frequencies can be tuned by a factor of 2 with tension induced by mechanical breakjunctions actuated by an electrostatic force or tension from bent suspended electrodes

Gate Controlled Photocurrent Generation Mechanisms in High-Gain In₂Se₃ Phototransistors

Photocurrent in photodetectors incorporating van der Waals materials is typically produced by a combination of photocurrent generation mechanisms that occur simultaneously during operation. Because of this, response times in these devices often yield to slower, high gain processes, which cannot be turned off. Here we report on photodetectors incorporating the layered material In₂Se₃, which allow complete modulation of a high gain, photogating mechanism in the ON state in favor of fast photoconduction in the OFF state. While photoconduction is largely gate independent, photocurrent from the photogating effect is strongly modulated through application of a back gate voltage. By varying the back gate, we demonstrate control over the dominant mechanism responsible for photocurrent generation. Furthermore, because of the strong photogating effect, these direct-band gap, multilayer phototransistors produce ultrahigh gains of (9.8 ± 2.5) × 10⁴ A/W and inferred detectivities of (3.3 ± 0.8) × 10¹³ Jones, putting In₂Se₃ among the most sensitive 2D materials for photodetection studied to date